Journal of Physics B Atomic Molecular and Optical Physics

INTRODUCTORY REVIEW

Cold molecules a chemistry kitchen forphysicistsTo cite this article Olivier Dulieu et al 2006 J Phys B At Mol Opt Phys 39 E01

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INSTITUTE OF PHYSICS PUBLISHING JOURNAL OF PHYSICS B ATOMIC MOLECULAR AND OPTICAL PHYSICS

J Phys B At Mol Opt Phys 39 (2006) doi1010880953-40753919E01

INTRODUCTORY REVIEW

Cold Molecules a chemistry kitchen for physicists

Olivier Dulieu1 Maurice Raoult1 and Eberhard Tiemann2

1 Laboratoire Aime Cotton CNRS Bat 505 Campus drsquoOrsay 91405 Orsay Cedex France2 Department of Quantum Optics Gottfried Wilhelm Leibniz Universitat HannoverWelfengarten 1 30167 Hannover Germany

E-mail olivierdulieulacu-psudfr tiemanniqouni-hannoverde

If you listen to physicists talking about molecules and chemistry you will often be imposedby words like lsquomolecules are too difficultrsquo or lsquomolecules contain too many atomsrsquo and lsquotheresearch routes in chemistry are too complex for overlapping them with the physical languagersquoThis statement should not be accepted because it would then have the same implications forconnections to biology and medical sciences Happily physicists have the long experience thatthe good way out of this dilemma is to set up models where both the system under study andits dynamics are simplified by lowering the number of degrees of freedom and by identifyingthe main interactions Physicists turn to small molecules like diatomics or triatomics whichare the building blocks for elementary chemical processes The availability of ensembles ofcold molecules which was initiated almost ten years ago now offers the exciting opportunityto simplify the dynamics by reducing the number of reaction channels to exactly one in theextreme case or by reaching a regime of lsquochemistry without entropyrsquo (sometimes labelledas lsquosuperchemistryrsquo [1]) meaning that coherences of quantum mechanics are the dominatingroutes of the processes

The present special issue intends to provide an instantaneous picture of the research oncold molecules in 2006 through a collection of 32 experimental and theoretical papers writtenby groups with long experience and by new ones in the field It focuses on the numerous recipeswhich are now available or proposed for creating cold molecules of various species and invarious environments Several papers illustrate the amazingly detailed knowledge which canbe extracted from their study opening the way to the ultimate control of elementary chemicalprocesses by mastering both the preparation of the initial state and the reaction path In thefollowing we will review the main stages of the developments of research on cold moleculesas reflected by the accompanying series of papers

This issue is motivated by a series of recent workshops held within the framework ofthe Research Training Network lsquoCold Moleculesrsquo of the European Commission [2] and ofthe network lsquoCollisions in Atom Trapsrsquo of the European Science Foundation [3] The mostrecent event was the Training School and Workshop lsquoAchievements and Perspectives for ColdMoleculesrsquo held in March 2006 in one of the lsquocoolestrsquo places in the French Alps the lsquoCentrede Physiquersquo of les Houches [4] While the following articles are not the proceedings of thisworkshop most of this work was presented there

Introductory Review

The early times

Cold molecule research began about twenty years ago initially with the availability of coldatomic gases now routinely created in many laboratories around the world using differentrecipes This progress and the related theory actually define the current standard vocabularyon which we will rely in the following The thermodynamic temperature T of a gaseoussample of structureless particles is linked to the average kinetic energy E of the particlesthrough the Boltzmann constant kB according to E = 3kBT2 By convention the coldregime is usually reached when T lt 1 K and the ultracold regime when T lt 1 mK Howeverthe word temperature is used even if the sample is not in thermodynamic equilibrium as thetemperature value represents a convenient scale for the description of the kinetic energy of theparticles It is worthwhile to mention for instance that the kinetic energy at 1 mK correspondsto 20 MHz in the frequency domain because many results within the field of cold molecules areclosely related to laser spectroscopy We will use atomic units in the following especially forinterparticle distances eg the Bohr radius a0 (a0 = 00529177 nm) except where otherwisestated

Lowering the velocity of atoms down to unprecedented limits leaving them almost atrest yields dilute samples which are sensitive to the weakest residual interactions present intheir surroundings Major achievements using ultracold atomic gaseous samples have beenobtained on atom optics (demonstrating the reproducibility of light optics phenomena withatoms up to the atom laser or designing new devices for control of atomic motion) onquantum degeneracy (probing Bose and Fermi statistics with weakly interacting particles)or on precision measurements (dramatically improving time and frequency standards) It isremarkable that within five years two Nobel Prizes were awarded to scientists in the field ofultracold matter [5 6 7 8 9]

In the early developments of atom cooling the interactions between atoms alreadyappeared producing loss channels in the traps despite the highly diluted ensembles but openingan entirely new field of research Extensive studies on cold atom collisions have been performed[10] enlightening a new collision regime the relative kinetic energy is so small that thecollision is dominated by the atomndashatom interaction at large distance R For instance caesiumatoms at 100 μK collide with a relative velocity of about 1 cm sminus1 while their van der Waalsinteraction (proportional to Rminus6 for non-degenerate ground state atoms) reaches a magnitudeof 100 μK around R asymp 140a0 Moreover the collision complex is almost not rotating iejustifying the condition of s-wave scattering as the centrifugal barrier of non-zero rotation canbe hardly overcome by the low kinetic energy of the atoms A richness of scattering resonancesappears due to atomic hyperfine interaction alone and to the competition with Zeeman energyinduced by external magnetic fields either present from the trap design or tuned on purpose toa desired value Early studies have been often devoted to the measurement of loss rates insideatom traps because collisions involving ground state atoms andor excited atoms intrinsicallylimit the maximum atomic density in the traps Several combinations of alkali atoms have beeninvestigated so far either homonuclear [10] or heteronuclear ones [11 12 13] The paper byAUBOCK et al from Graz reports on one of the missing pairs involving the lightest alkali atomsLi and Na They investigate the influence of the atom loss from one species induced by thepresence of the other species All such results are important for the optimization of mixedtraps

The lsquophotoassociative panrsquo assembling cold atoms with light

Interactions between atoms control molecule formation in a cold ensemble Studieson molecule creation from cold atoms started as early as 1987 with the proposal of

Introductory Review

photoassociation (PA) of cold atoms [14] and with the first observation in 1993 with coldsodium [15] and rubidium atoms [16] The PA process is actually a collision assisted bylight where the colliding pair of atoms absorbs a photon with suitable frequency to create anexcited and in most cases short-lived molecule in a well-defined rovibrational bound levelThis free-bound process represents a high resolution spectroscopy as the kinetic energy spreadof the atoms is smaller than the energy spacing between molecular bound levels and moreimportantly very often narrower than the level width of the excited states When studied ona single species trap the PA laser is regularly chosen so that its frequency is detuned fromthe frequency of the strongest SndashP atomic transition and the excited atom pair experiencesthe Rminus3 long-range dipolendashdipole interaction Thus PA is very efficient in this case at largedistances (typically beyond 100a0) and excites levels close to the dissociation limit havinglarge amplitude vibrational motion In mixed species traps atoms of the two species interactthrough the shorter range van der Waals interaction (varying as Rminus6) PA then occurs at shorterinteratomic distances (R lt 50a0) and is much less efficient than in the homonuclear case forcomparable temperature and density conditions [17]

The wealth of results obtained on photoassociation is well documented in several reviewpapers [18 19 20] PA has been observed for many atomic species which are convenientlylaser-cooled like alkalis (Li Na K Rb Cs) alkaline-earth (Ca Sr) helium hydrogen andytterbium Due to its resonant character PA has been widely used as a high resolutionspectroscopic technique to investigate long-range molecular states In particular purelylong-range molecular states in alkali dimers have been probed with this technique Theyexhibit potential wells entirely located outside the range of chemical bonding [21] due to thecompetition between atomic spinndashorbit and dipolendashdipole interactions Through the latter veryaccurate atomic radiative lifetimes have been determined

Four papers in this issue illustrate the power of PA spectroscopy to investigate dynamicaleffects within the molecules or induced by light interaction with the atomic pair BERGEMAN etal and PICHLER et al interpret observed perturbations in the photoassociated molecules createdin high vibrational levels of their electronically excited states (ie the atoms mostly moving atlarge distances) induced by the molecular spinndashorbit coupling mainly acting at short distances(around 10a0) In both cases the interplay with theoretical models has proved a guiding toolfor the interpretation

Metastable helium atoms (in their lowest level 23S) represent a system of choice forPA because of the simplification of the physical system by quantum numbers and long rangecharacter Indeed in contrast with alkali atoms helium has no hyperfine structure so that thereare only few possible reaction channels which can all be taken into account in a simple modelThe domain of validity of photoassociation theories [22 23] can be checked very preciselyby comparing for instance the calculated light shifts and light broadening to the experimentalones as demonstrated by the paper of PORTIER et al They analysed experimental data of aprevious paper of their own where the photoassociation of spin polarized metastable He intolevels of a really long-range (asymp 150a0) molecular potential well of the asymptote 23S+23P0

was observed preventing the system from fast ionization The relation of the light inducedeffects to the scattering length of the lsquogroundrsquo state 23S was derived The accompanying paperby VAN DER ZWAN et al from the Utrecht group focuses on the role of Penning ionization inthe photoassociation spectra at the asymptote 23S+23P2 of He By designing coupled channelmodels they get the correct description of the observed resonances but the line profiles needrefinement of the models

Introductory Review

lsquoWell-cookedrsquo ultracold molecules stabilization

Stable ultracold molecules are needed to work with them during long times typically ofthe order of the trapping lifetime The lsquophotoassociative panrsquo produces ultracold excitedmolecules which tend to decay radiatively into pairs of hot atoms escaping from the lsquopanrsquo(or more physically from the atom trap) This kind of heating process can be turned intoan internal cooling process by choosing the right ingredients in order to enhance the decayof photoassociated molecules into ultracold long-lived molecules In 1997 the Orsay groupobserved for the first time such molecules starting from ultracold caesium atoms [24] thanksto both the peculiar properties of caesium dimers and to the detection method

The caesium dimer is one of two known molecules (the rubidium dimer is the other)whose purely long-range potential well reaches distances which are short enough to allow anefficient decay towards strongly bound levels of the lowest electronic states More preciselythe amplitude of the photoassociated wave function has to have significant values both at largedistances for the PA step and at short distance for the decay step These molecules are thendetected by two-photon resonant ionization yielding molecular ions which are easily collectedwith time-of-flight mass spectrometry As expected the same result was obtained shortly afterwith ultracold rubidium atoms [25] Another formation path through photoassociation is basedon the radial interaction between two electronic states and has been probed by Dion et al [26]on caesium PA This so-called lsquoresonant couplingrsquo relies on non-BornndashOppenheimer effectswhich are known to be present in almost all diatomic molecules and which are the cause forthe perturbations discussed in the papers by BERGEMAN et al and PICHLER et al in this issue Forthe lighter alkali dimers the task is not so easy as the long-range wells are located at too largedistances or the level densities do not favour resonant coupling Fatemi et al [27] from NISTwere however successful in observing with one-photon PA ultracold Na2 molecules in theirhighest vibrational levels much in the way which was originally proposed by Thorsheim et al[14] at the very end of their pioneering paper Nikolov et al from the University of Connecticut[28] were also able to detect a few ultracold K2 molecules exploiting the detailed knowledgeof the K2 structure supplied by conventional high resolution spectroscopy Shortly after thesame group demonstrated a more efficient formation scheme relying on two-step PA of coldpotassium atoms [29] theoretically worked out by Band and Julienne [30]

The new paper by HUANG et al from the University of Connecticut shows that the PA stepand the detection step are now so well mastered that they give access to the spectroscopy ofthe ultracold Rb2 molecules created in their absolute ground state X1+

g This is particularlyrelevant for optimizing the PA transitions which would favour the formation of ultracoldmolecules in low vibrational levels and ultimately in the lowest v = 0 level With a similargoal VATASESCU et al investigate the tunnelling effect in a double well potential of Cs2 whichis thought to be responsible for anomalous features in the PA spectrum of ultracold caesiumand predicted efficient production of ultracold Cs2 molecules in a fairly narrow distribution oflow vibrational levels (around v = 7) of the metastable a3+

u Let us note that HUANG et al(as well as the similar study of [31]) also demonstrate that the spectroscopy of the molecularstates involved in the first step of the two-photon resonant detection can be precisely assigned

The heteronuclear alkali diatomic molecules appear naturally as further candidates forPA and cold molecule formation These systems belong to the class of molecules commonlyreferred to as polar molecules characterized by a permanent electric dipole moment whichinduces an anisotropic long-range interaction between molecules Such a feature is predictedto reveal new effects in quantum degenerate gases [32] to be a building block of a molecularquantum information devices [33] or to design new setups for molecular optics [34] Heavymolecules with large permanent electric dipole moment are also considered as good candidates

Introductory Review

for the quest of the dipole moment of the electron [35 36 37] which represents a stringenttest of the standard model An overview of recent developements on cold polar moleculescan be found in [38] As discussed in [17 39] the short range Rminus6 variation of the atomndashatom interaction in the excited molecular states weakens photoassociation of mixed alkali pairscompared to the homonuclear ones On the other hand Azizi et al [17] demonstrated that thedecay back to the ground state or to the lowest triplet state is enhanced as the correspondingmolecular potential curves also behave as Rminus6 The overall rates for ultracold moleculeformation from mixed pairs are predicted to be only 10 to 50 times smaller than for homonuclearpairs This has been confirmed by the results obtained with RbCs by Kerman et al [40] and withKRb by Wang et al [41] In both cases the vibrational population of the ultracold moleculeshas been understood [42 43] and the observation of RbCs molecules in v = 0 has even beenreported [42]

The paper by WANG et al uses the advantage that ultracold molecules are produced in wellidentified vibrational levels with only few rotational levels populated by the photoassociationand spontaneous-decay sequence This simplifies the spectroscopy which otherwise would bevery complex because of the absence of inversion symmetry (ie the absence of a selectionrule associated with the gerade or ungerade character) in heteronuclear dimers more molecularstates are expected to interact together leading to complicated molecular level structure ThusWANG et al were able to analyse previously unknown molecular states of KRb confirmingtheoretical predictions and opening ways for new molecular state preparation

The work of HAIMBERGER et al from Rochester adds one more species of ultracoldmolecules NaCs following their preliminary study [12] In contrast with RbCs and KRbthe NaCs molecule has a large permanent dipole moment provided one can prepare lowvibrational levels of the ground state for further studies The group in Freiburg (KRAFT etal) also reports on their very recent observation of ultracold LiCs molecules which have thelargest dipole moment of all known alkali pairs This study is only at its beginning in contrastwith the other papers the PA step is not provided by a separate laser but by the trapping laserjust as Mancini et al observed ultracold KRb molecules [13] for the first time

It is worth noting that these successes are also built on accurate knowledge of the structureof these molecules brought by conventional high resolution spectroscopy over many yearsThe group at Hannover is a well-recognized specialist for this technique designed to producedata relevant for ultracold collisions ie the knowledge of levels close to the dissociation limitof the ground state in order to deduce scattering properties of the corresponding atom pairThis is illustrated as a nice complement to the work by HAIMBERGER et al with the paper onthe pair Na + Cs from DOCENKO et al the coupling between the singlet ground and the tripletmetastable state due to hyperfine interaction is fully modelled from a large data set of observedfluorescence spectra and predictions of Feshbach resonance are presented

Two theoretical papers explore new species and possible new mechanisms for ultracoldmolecule production AYMAR et al at CNRS-Orsay investigate the possibility of formingmolecules composed with francium and Rb or Cs motivated by the availability of cold franciumtraps which will be mixed in the near future with Rb or Cs traps [44 45] Using new quantumchemistry calculations of francium for computing potential curves they point out that theunusually large fine structure of francium plays an important role in the question of existingdouble well potentials which were helpful in producing ultracold Cs2 for the first time [24]

JUARROS et al at the Centre for Astrophysics at Harvard are interested in the formation of oneof the favourite molecules of astrophysicists namely LiH opening in a theoretical feasibilitystudy a different class of molecules Assuming sufficiently dense ultracold ensembles of Liand H they give quantitative results for molecular ground state population via the B1 statewhich seems to be preferable compared to the A1+ state

Introductory Review

Time allows fine cooking processes under optimal control

The photoassociation route described above relies on cw lasers for the excitation step and onspontaneous emission for the stabilization step While producing large amounts of ultracoldmolecules the main drawback of this sequence is that ultracold molecules are generally createdin a broad distribution of vibrational levels most often well above the v = 0 level Inthis respect the situation described by VATASESCU et al is probably an exceptional case Analternative has been explored through a two-colour stimulated Raman scheme [46 47 48] butthe efficiency of the process is severely limited by the reversibility of the process ie theultracold molecules can be dissociated through the same scheme

The idea of using laser pulses to overcome this limitation rose some time ago in thecontext of cold atom photoassociation with the model proposed by Machholm et al [49]based on a pumpndashdump scheme It is easy to realize that the characteristics of the pulses egintensity duration time delay offer control parameters to optimize the process efficiency verymuch along the ideas of adaptive optimal control initiated by Judson and Rabitz [50] Morerecently chirped laser pulses have been considered theoretically for optimizing the formationof ultracold molecules in a pumpndashdump scheme [51 52] None of these possibilities have beenexperimentally demonstrated yet even if a certain degree of control of cold collisions [53] andof dissociation of ultracold Rb2 molecules with chirped laser pulses [54] has been observed aswell as of ultracold Rb2 dissociation with feedback control [55]

Four theoretical papers in this issue intend to model possible experiments with Rb2 dimersas this is the species chosen by the experimental groups above The paper by KOCH et alassesses the feasibility of a formation scheme with picoseconds chirped lasers by calculatingthe number of created molecules per pulse with three different models for taking in account theinitial velocity distribution of the atoms POSCHINGER et al and BROWN AND WALMSLEY explorecomplementary ways to design shaped femtosecond pulses the former uses adaptive feedbackcontrol of the pulse amplitude and phase with a genetic algorithm while the latter focuses onintuitive pulse designs which could provide a starting point for a future experiment FinallySCHAFER-BUNG et al assume that the PA step is achieved via cw lasers creating an excited Rb2

molecule in a well-defined vibrational level and concentrate on the stabilization step througha pulse sequence designed by an optimal control procedure

In the absence of experiments approaching the proposed models above it is too early tocomment on the quality of these models However the theoretical investigations undoubtlyinclude more and more refinements which will certainly be very useful for guiding upcomingexperiments

lsquoRefrigeratingrsquo molecules

The laser schemes are very efficient in cooling but are not generally applicable at least formolecules For atoms the limits mainly come from the possible unavailability of laser sourcesfor exciting a good atomic transition But for molecules the wide spread of the spontaneousdecay into different quantum states limits the process to a few cooling cycles before themolecule is lost in a state which is not in resonance with the applied field even if one overlapsseveral light fields Chirped lasers are still to be explored for this application

As in solid state physics several cooling steps could progress to the desired goal ofultracold ensembles As a first step buffer-gas cooling is an idea which was successfullyapplied for the molecule CaH by the Doyle group at Harvard [56] which because of itsparamagnetic ground state could be magnetically trapped at temperatures around 100 mKOther molecules could be cooled with this method but not trapped so far In their paper

Introductory Review

BAKKER et al from the Berlin group of A Peters report a new setup for buffer-gas cooling andcould successfully demonstrate the cooling and trapping of Cr atoms The paper discusses indetail the necessary conditions for trapping paramagnetic molecules after buffer-gas coolingand proposes as good candidates CrH and MnH with their high spin ground states and not toolarge spinndashspin interactions

A very different cooling device for molecules is the helium nanodroplet expanding out ofa nozzle which equilibrates around a few tenths of a Kelvin The beam of droplets consistingof thousands of He atoms travels through a pickup cell where the atoms and molecules areloaded onto the droplet which will equilibrate again by vaporizing He atoms Under thesecircumstances metastable molecular states are formed as is demonstrated for alkali dimersALLARD et al from Graz report on new experiments on Rb2 and KRb where they observethe lowest triplet state of desorbing molecules from the droplet and additionally get the firstinformation about the distribution of populated molecular levels from the desorption Theaccompanying paper by BEUC et al from Zagreb gives valuable predictions for the expectedband profiles of absorption spectra from the metastable triplet state of KRb under the conditionsof such cold He droplets and will guide further experiments such as those from Graz The Hedroplets are a quite universal cold laboratory for a large manifold of molecules The groupin Rostock (see the paper by PRZYSTAWIK et al) applies it for silver dimers and reports on thebuilding of the metastable triplet state of Ag2 which is very interesting because Ag2 is locatedinside the He droplet whereas the alkali dimers sit on the surface of the droplet Thereforevery different interactions between the molecule and the droplet are expected This question isin view by the work of the group of F Stienkemeier at Freiburg (see the paper by CLAAS et al)where they studied K2 on the droplet and used the pumpndashprobe technique with femtosecondlaser pulses They find differences between the dynamics of photoionization in the gas phaseand on the droplet Also the timescale of the desorption process is identified

Supersonic expansion of gases other than He can lead to beams cooled in the translationalvelocity spread and internal energy to temperatures below 1 K Such cold samples are studiedfor weakly bound systems like van der Waals molecules KOPERSKI AND FRY apply this techniquefor spectroscopic studies of some metal noble gas compounds and especially to Hg2 for whichspectroscopic data are valuable for photoassociation experiments of trapped Hg atoms

Refrigerating is not universal slowing molecules

Parallel to the intensive studies of laser cooling of atoms like alkalis alkaline-earth elements anda few others or to the application of a coolant to widen the scope of molecules researchers haveaimed at creating slow molecules starting from fast molecular beams The main achievementsconcern the Stark deceleration technique [57] and the selection of slow molecules out of thefast ones [58] using guiding electric quadrupole fields With such techniques polar moleculeslike ND3 OH or CH2CO were successfully decelerated or selected Let us note that the heavypolar molecule YbF has been slowed down using the Stark deceleration approach for the questof permanent electric dipole moment of the electron [59]

New advances are reported by VAN DER MEERAKKER et al at the FHI in Berlin deceleratingNH in its metastable state a1 then converted inside a magnetic trap to the ground state X3minusby which accumulation of several decelerated packages could be achieved Additionally theypropose a nice idea for compressing the phase space by dumping to the ground state witha cw laser instead of a pulsed laser naturally assumed for packages arriving periodically intime With light fields it is not very difficult to obtain huge electric field strength by focussingThus in the paper by FULTON et al from Edinburgh deceleration is studied with a periodicoptical lattice from pulsed laser fields for NO and benzene reducing the kinetic energy by

Introductory Review

as much as 75 of the incoming one This might open research to much wider classes ofmolecules The paper by JUNG et al brings applications of the decelerated SO2 molecule intoview They measured the electric dipole moment of an excited state which they propose to usein a photodissociation experiment to produce cold radicals such as SO and O and the electricdipole moment could allow tuning of the photodissociation and thus the kinetic energy of thefragments

New recipes

After the fast development of different cooling recipes we need recipes and concepts forapplications With ultracold atomic ensembles in optical lattices one is able to model differentvariants of the BosendashHubbard system eg the transition from the Mott-insulator to thesuperfluid phase and the creation of ultracold dipolar molecular gases will add an importantnew ingredient by the anisotropy of the long range interaction to the richness of possible phasetransitions

But ultracold molecules opened the presently emerging field of ultracold chemistry witheither quantum effects (superchemistry with degenerate gases) and control of interactions withexternal fields (laser or static magnetic and electric fields) being new recipes in chemistry Firstexperimental observations of ultracold atomndashmolecule reactions have been reported [60 61]studying the lifetime of Cs2 ensembles embedded in Cs ensembles More theoretical modelsare required with which good experimental systems can be proposed or guided in future Thetheoretical work by WECK AND BALAKRISHNAN gives a general introduction to the physicalsituation of atomndashmolecule reactions at threshold and emphasizes the importance of theappearance of resonances otherwise low reaction rates are normally expected In the paper byYANG et al the link between ultracold collisions of atoms and molecules for quenching and theexperiments of buffer gas cooling mentioned above is presented for the example of rotationalquenching of CO through collision with H He or H2 in a wide range of energies from 10 μKto about 10 K as needed in the different stages of cooling processes The paper by GONZALEZ-SANCHEZ et al reports on similar aspects on the even simpler system OHminus in its ground state1+ with He atoms in 1S0 to be as complete as possible in the theoretical modelling Allthese calculations give rate constants which are of a magnitude to be observable in upcomingexperiments ultracold chemistry is probably richer than is thought by many chemists

Ion crystals but not traditional salt of the kitchen

Ion reactions play an important role in chemistry and for the cold regime the kitchen withinthe cold interstellar clouds comes immediately into view as a highly diluted system Thustrapped molecular ions certainly belong to a representation of the status of research of coldmolecules and their applications The group at Dusseldorf demonstrates in the paper by ROTH

et al the general applicability of sympathetic cooling of reaction partners like noble gas ionsor N+

2 and O+2 by laser cooled Be+ ions to produce a large variety of new cold ion samples eg

H+3 N2H+ etc These may serve for high-precision measurements for fundamental physical

questions like the time variation of fundamental constants or for studies of reaction processesrelevant in interstellar chemistry

Because of the strong Coulomb interaction between ions they appear regularly as Coulombcrystals if sufficiently cooled which gives great advantages because the ions within a crystalare well localized and thus addressable for particle by particle interaction and single particledetection To be quantum state specific in such studies the preparation of the initial physical

Introductory Review

condition is of fundamental importance This problem is addressed by two papers of theDrewsen group at Arhus In the first one by VOGELIUS et al they theoretically study the case ofrotational cooling of a polar molecule (here an ion which has an electric dipole moment withreference to its centre of mass) by optical coupling of the collective modes of a two-ion systemAs an example they describe the situation of MgH+ which is easily sympathetically cooled byCa+ In a second paper by the same authors they explore the possibility for probabilistic statepreparation in single molecular ions by projection measurements

Conclusion

Cold molecules offer a fantastic opportunity to develop strongly interdisciplinary research Twodomains are presently in view condensed matter physics with strongly correlated particlesand chemistry and superchemistry with the clear appearance of quantum effects in reactionprocesses This special issue on lsquoCold Moleculesrsquo gives insight into present lsquocookingrsquo inthe different laboratories which still concentrates on developing new methods and schemesto eventually proceed to systems which are most appropriate for the physical and chemicalquestions and applications When research on cold molecules started many scientists thought itwill be an artificial side-field of short duration But looking back the applicability of ultracoldatomic ensembles to model ideal solid state physics was not obvious Similarly the possiblerichness of superchemistry is unexplored but calculations of some systems clearly indicate thatthere is great hope of exciting results To harvest the fruits is probably more difficult becauseof the large number of degrees of freedom which we have to address by elegant experimentaland theoretical ideas and techniques not to be lost in the overwhelming routes which naturein principle has at its disposal and selects by evolution during millions of years We have torecognize the finite human lifetime Thus a thorough study of the papers provided by excellentlaboratories will be the right way of developing ideas during the short time available to us Wethank the authors for their contributions

Acknowledgments

The authors of this editorial acknowledge support from the Research and training Networkof the European Commission lsquoCold Moleculesrsquo (Contract HPRN- CT-2002-00290) and ODfrom the European Science Foundation through the network lsquoCollisions in Atom trapsrsquo

References

[1] Heinzen D J Wynar R Drummond P D and Kheruntsyan K V 2000 Superchemistry Dynamics of coupledatomic and molecular bose-einstein condensates Phys Rev Lett 84 5029

[2] httpwwwlacu-psudfrcoldmolecules[3] httpphysicsopenacukcats[4] httpquantendynamikphysikuni-freiburgdeles houches 2006[5] Chu S 1998 Nobel lecture the manipulation of neutral particles Rev Mod Phys 70 685[6] Cohen-Tannoudji C N 1998 Nobel lecture manipulating atoms with photons Rev Mod Phys 70 707[7] Phillips W D 1998 Nobel lecture laser cooling and trapping of neutral atoms Rev Mod Phys 70 721[8] Cornell E A and Wieman C E 2002 Nobel lecture BosendashEinstein condensation in a dilute gas the first 70 years

and some recent experiments Rev Mod Phys 74 875[9] Ketterle W 2002 Nobel lecture when atoms behaves as waves BosendashEinstein condensation and the atom laser

Rev Mod Phys 74 1131[10] Weiner J Bagnato V S Zilio S C and Julienne P S 1999 Experiments and theory in cold and ultracold collisions

Rev Mod Phys 71 1

Introductory Review

[11] Schloder U Engler H Schunemann U Grimmand R and Weidemuller M 1999 Cold inelastic collisions betweenlithium and cesium in a two- species magneto-optical trap Eur Phys J D 7 331

[12] Shaffer J P Chalupczak W and Bigelow N P 1999 Highly-excited states of ultracold molecules photoassociativespectroscopy of Na2 Phys Rev Lett 83 3621

[13] Mancini M W Telles G D Caires A R L Bagnato V S and Marcassa L G 2004 Observation of ultracoldground-state heteronuclear molecules Phys Rev Lett 92 133203

[14] Thorsheim H R Weiner J and Julienne P S 1987 Laser-induced photoassociation of ultracold sodium atomsPhys Rev Lett 58 2420

[15] Lett P D Helmerson K Philips W D Ratliff L P RolstonS L and Wagshul M E 1993 Spectroscopy of Na2 byphotoassociation of laser-cooled Na Phys Rev Lett 71 2200

[16] Miller J D Cline R A and Heinzen D J 1993 Photoassociation spectrum of ultracold Rb atoms Phys Rev Lett71 2204

[17] Azizi S Aymar M and Dulieu O 2004 Prospects for the formation of ultracold ground state polar moleculesfrom mixed alkali atom pair Eur Phys J D 31 195

[18] Lett P D Julienne P S and Philips W D 1995 Photoassociative spectroscopy of laser-cooled atoms Annu RevPhys Chem 46 423

[19] Stwalley W C and Wang H 1999 Photoassociation of ultracold atoms a new spectroscopic technique J MolSpectrosc 195 194

[20] Jones K M Tiesinga E Lett P D and Julienne P S 2006 Ultracold photoassociation spectroscopy Long-rangemolecules and atomic scattering Rep Prog Phys 78 483

[21] Stwalley W C Uang Y H and Pichler G 1978 Pure long-range molecules Phys Rev Lett 41 1164[22] Bohn J L and Julienne P S 1996 Semianalytic treatment of two-color photoassociation spectroscopy and control

of cold atoms Phys Rev A 54 R4637[23] Bohn J L and Julienne P S 1999 Semianalytic theory of laser-assisted resonant cold collisions Phys Rev A 60

414[24] Fioretti A Comparat D Crubellier A Dulieu O Masnou-Seeuws F and Pillet P 1998 Formation of Cs2 cold

molecules through photoassociation Phys Rev Lett 80 4402[25] C Gabbanini A Fioretti A Lucchesini S Gozzini and M Mazzoni 2000 Cold rubidium molecules formed in a

magneto-optical trap Phys Rev Lett 84 2814[26] Dion C M Drag C Dulieu O Laburthe Tolra B Masnou-Seeuws F and Pillet P 2001 Resonant coupling in the

formation of ultracold ground state molecules via photoassociation Phys Rev Lett 86 2253[27] Fatemi F K Jones K M Lett P D and Tiesinga E 2002 Ultracold ground-state molecule production in sodium

Phys Rev A 66 053401[28] Nikolov A N Eyler E E Wang X T Li J Wang H Stwalley W C and Gould P L 1999 Observation of ultracold

ground state potassium molecules Phys Rev Lett 82 703[29] Nikolov A N Enscher J R Eyler E E Wang H Stwalley W C and Gould P L 2000 Efficient production of ground

state potassium molecules at sub-mK temperatures by two-step photoassociation Phys Rev Lett 84 246[30] Band Y B and Julienne P S 1995 Ultracold molecule production by laser-cooled atom photoassociation Phys

Rev A 51 R4317[31] Lozeille J Fioretti A Gabbanini C Huang Y Pechkis H K Wang D Gould P L Eyler E E Stwalley W C

Aymar M and Dulieu O 2006 Detection of cold Rb2 triplet state molecules by two-photon ionization EurPhys J D 39

[32] Damski B Santos L Tiemann E Lewenstein M Kotochigova S Julienne P and Zoller P 2003 Creation of adipolar superfluid in optical lattices Phys Rev Lett 90 110401

[33] DeMille D 2002 Quantum computation with trapped polar molecules Phys Rev Lett 88 067901[34] Kallush S Segev B and Cote R 2005 Evanescent-wave mirror for ultracold diatomic polar molecules Phys Rev

Lett 95 163005[35] Sandars P G H 1967 Measurability of th eproton electric dipole moment Phys Rev Lett 19 1396[36] Hudson J J Sauer B E Tarbutt M R and Hinds E A 2002 Measurement of the electron electric dipole moment

using YbF molecules Phys Rev Lett 89 023003[37] Kawall D Bay F Bickman S Jiang Y and DeMille D 2004 Precision ZeemanndashStark spectroscopy of the

metastable a(1)3σ + state of PbO Phys Rev Lett 92 133007[38] Doyle J Friedrich B Krems R V and Masnou-Seeuws F 2004 Special issue on ultracold polar molecules 2004

Eur Phys J D 31 149 and following papers[39] Wang H Gould P L and Stwalley W C 1998 Fine-structure predissociation of ultracold photoassociated39K2

molecules observed by fragmentation spectroscopy Phys Rev Lett 80 476[40] Kerman A J Sage J M Sainis S Bergeman T and DeMille D 2004 Production and state-selective detection of

ultracold rbcs molecules Phys Rev Lett 92 153001

Introductory Review

[41] Wang D Qi J Stone M F Nikolayeva O Wang H Hattaway B Gensemer S D Gould P L Eyler E E andStwalley W C 2004 Photoassociative production and trapping of ultracold krb molecules Phys Rev Lett 93243005

[42] Sage J M Sainis S Bergeman T and DeMille D 2005 Optical production of ultracold polar molecules PhysRev Lett 94 203001

[43] Wang D Qi J Stone M F Nikolayeva O Hattaway B Gensemer S D Wang H Zemke W T Gould P LEyler E E and Stwalley W C 2004 The photoassociative spectroscopy photoassociative molecule formationand trapping of ultracold 39K85Rb Eur Phys J D 31 165

[44] Orozco L 2006 private communication[45] Sprouse G D Fliller R P Grossman J S Orozco L A and Pearson M R 2002 Traps for neutral radioactive atoms

Nucl Phys A 701 597[46] Abraham E R I McAlexander W I Sackett C A and Hulet R G 1995 Spectroscopic determination of the s-wave

scattering length of lithium Phys Rev Lett 74 1315[47] Tsai C C Freeland R S Vogels J M Boesten H M J M Gardner J R Heinzen D J and Verhaar B J 1997

Two-color photoassociation spectroscopy of ground state Rb2 Phys Rev Lett 79 1245[48] Laburthe Tolra B Drag C and Pillet P 2001 Formation of cold cesium molecules through stimulated raman

photoassociation Phys Rev A 64 R61401[49] Machholm M Giusti-Suzor A and Mies F H 1994 Photoassociation of atoms in ultracold collisions probed

bywave-packet dynamics Phys Rev A 50 5025[50] Judson R and Rabitz H 1992 Teaching lasers to control molecules Phys Rev Lett 68 1500[51] Vala J Dulieu O Masnou-Seeuws F and Kosloff R 2001 Coherent control of cold molecule formation through

photoassociation using a chirped pulsed laser field Phys Rev A 63 013412[52] Koch C P Luc-Koenig E and Masnou-Seeuws F 2006 Making ultracold molecules in a two color pump-dump

photoassociation scheme using chirped pulses Phys Rev A 73 033408[53] Wright M J Gensemer S D Vala J Kosloff R and Gould P L 2005 Control of ultracold collisions with frequency-

chirped light Phys Rev Lett 95 063001[54] Brown B L Dicks A J and Walmsley I A 2006 Coherent control of ultracold molecule dynamics in a magneto-

optical trap by use of chirped femtosecond laser pulses Phys Rev Lett 96 173002[55] Salzmann W Poschinger U Wester R Weidemuller M Merli A Weber S M Sauer F Plewicki M Weise F

Esparza A M Woste L and Lindinger A 2006 Coherent control with shaped femtosecond laser pulses appliedto ultracold molecules Phys Rev A 73 023414

[56] Weinstein J D deCarvalho R Kim J Patterson D Friedrich B and Doyle J M 1998 Magnetic trapping of atomicchromium Phys Rev A 57 R3173

[57] Bethlem H L and Meijer G 2003 Production and application of translationally cold molecules Int Rev PhysChem 22 73

[58] Rangwala S A Junglen T Rieger T Pinkse P W H and Rempe G 2003 Contnuous source of translationally colddipolar molecules Phys Rev A 64 043406

[59] Tarbutt M R Bethlem H L Hudson J J Ryabov V L Ryzhov V A Sauer B E Meijer G and Hinds E A 2004Slowing heavy ground-state molecules using an alternating gradient decelerator Phys Rev Lett 92 173002

[60] Staanum P Kraft S D Lange J Wester R and Weidemuller M 2006 Experimental investigation of ultracoldatom-molecule collisions Phys Rev Lett 96 023201

[61] Zahzam N Vogt T Mudrich M Comparat D and Pillet P 2006 Atomndashmolecule collisions in an optically trappedgas Phys Rev Lett 96 023202

Introductory Review

photoassociation (PA) of cold atoms [14] and with the first observation in 1993 with coldsodium [15] and rubidium atoms [16] The PA process is actually a collision assisted bylight where the colliding pair of atoms absorbs a photon with suitable frequency to create anexcited and in most cases short-lived molecule in a well-defined rovibrational bound levelThis free-bound process represents a high resolution spectroscopy as the kinetic energy spreadof the atoms is smaller than the energy spacing between molecular bound levels and moreimportantly very often narrower than the level width of the excited states When studied ona single species trap the PA laser is regularly chosen so that its frequency is detuned fromthe frequency of the strongest SndashP atomic transition and the excited atom pair experiencesthe Rminus3 long-range dipolendashdipole interaction Thus PA is very efficient in this case at largedistances (typically beyond 100a0) and excites levels close to the dissociation limit havinglarge amplitude vibrational motion In mixed species traps atoms of the two species interactthrough the shorter range van der Waals interaction (varying as Rminus6) PA then occurs at shorterinteratomic distances (R lt 50a0) and is much less efficient than in the homonuclear case forcomparable temperature and density conditions [17]

The wealth of results obtained on photoassociation is well documented in several reviewpapers [18 19 20] PA has been observed for many atomic species which are convenientlylaser-cooled like alkalis (Li Na K Rb Cs) alkaline-earth (Ca Sr) helium hydrogen andytterbium Due to its resonant character PA has been widely used as a high resolutionspectroscopic technique to investigate long-range molecular states In particular purelylong-range molecular states in alkali dimers have been probed with this technique Theyexhibit potential wells entirely located outside the range of chemical bonding [21] due to thecompetition between atomic spinndashorbit and dipolendashdipole interactions Through the latter veryaccurate atomic radiative lifetimes have been determined

Four papers in this issue illustrate the power of PA spectroscopy to investigate dynamicaleffects within the molecules or induced by light interaction with the atomic pair BERGEMAN etal and PICHLER et al interpret observed perturbations in the photoassociated molecules createdin high vibrational levels of their electronically excited states (ie the atoms mostly moving atlarge distances) induced by the molecular spinndashorbit coupling mainly acting at short distances(around 10a0) In both cases the interplay with theoretical models has proved a guiding toolfor the interpretation

Metastable helium atoms (in their lowest level 23S) represent a system of choice forPA because of the simplification of the physical system by quantum numbers and long rangecharacter Indeed in contrast with alkali atoms helium has no hyperfine structure so that thereare only few possible reaction channels which can all be taken into account in a simple modelThe domain of validity of photoassociation theories [22 23] can be checked very preciselyby comparing for instance the calculated light shifts and light broadening to the experimentalones as demonstrated by the paper of PORTIER et al They analysed experimental data of aprevious paper of their own where the photoassociation of spin polarized metastable He intolevels of a really long-range (asymp 150a0) molecular potential well of the asymptote 23S+23P0

was observed preventing the system from fast ionization The relation of the light inducedeffects to the scattering length of the lsquogroundrsquo state 23S was derived The accompanying paperby VAN DER ZWAN et al from the Utrecht group focuses on the role of Penning ionization inthe photoassociation spectra at the asymptote 23S+23P2 of He By designing coupled channelmodels they get the correct description of the observed resonances but the line profiles needrefinement of the models

Introductory Review

lsquoWell-cookedrsquo ultracold molecules stabilization

Stable ultracold molecules are needed to work with them during long times typically ofthe order of the trapping lifetime The lsquophotoassociative panrsquo produces ultracold excitedmolecules which tend to decay radiatively into pairs of hot atoms escaping from the lsquopanrsquo(or more physically from the atom trap) This kind of heating process can be turned intoan internal cooling process by choosing the right ingredients in order to enhance the decayof photoassociated molecules into ultracold long-lived molecules In 1997 the Orsay groupobserved for the first time such molecules starting from ultracold caesium atoms [24] thanksto both the peculiar properties of caesium dimers and to the detection method

The caesium dimer is one of two known molecules (the rubidium dimer is the other)whose purely long-range potential well reaches distances which are short enough to allow anefficient decay towards strongly bound levels of the lowest electronic states More preciselythe amplitude of the photoassociated wave function has to have significant values both at largedistances for the PA step and at short distance for the decay step These molecules are thendetected by two-photon resonant ionization yielding molecular ions which are easily collectedwith time-of-flight mass spectrometry As expected the same result was obtained shortly afterwith ultracold rubidium atoms [25] Another formation path through photoassociation is basedon the radial interaction between two electronic states and has been probed by Dion et al [26]on caesium PA This so-called lsquoresonant couplingrsquo relies on non-BornndashOppenheimer effectswhich are known to be present in almost all diatomic molecules and which are the cause forthe perturbations discussed in the papers by BERGEMAN et al and PICHLER et al in this issue Forthe lighter alkali dimers the task is not so easy as the long-range wells are located at too largedistances or the level densities do not favour resonant coupling Fatemi et al [27] from NISTwere however successful in observing with one-photon PA ultracold Na2 molecules in theirhighest vibrational levels much in the way which was originally proposed by Thorsheim et al[14] at the very end of their pioneering paper Nikolov et al from the University of Connecticut[28] were also able to detect a few ultracold K2 molecules exploiting the detailed knowledgeof the K2 structure supplied by conventional high resolution spectroscopy Shortly after thesame group demonstrated a more efficient formation scheme relying on two-step PA of coldpotassium atoms [29] theoretically worked out by Band and Julienne [30]

The new paper by HUANG et al from the University of Connecticut shows that the PA stepand the detection step are now so well mastered that they give access to the spectroscopy ofthe ultracold Rb2 molecules created in their absolute ground state X1+

g This is particularlyrelevant for optimizing the PA transitions which would favour the formation of ultracoldmolecules in low vibrational levels and ultimately in the lowest v = 0 level With a similargoal VATASESCU et al investigate the tunnelling effect in a double well potential of Cs2 whichis thought to be responsible for anomalous features in the PA spectrum of ultracold caesiumand predicted efficient production of ultracold Cs2 molecules in a fairly narrow distribution oflow vibrational levels (around v = 7) of the metastable a3+

u Let us note that HUANG et al(as well as the similar study of [31]) also demonstrate that the spectroscopy of the molecularstates involved in the first step of the two-photon resonant detection can be precisely assigned

The heteronuclear alkali diatomic molecules appear naturally as further candidates forPA and cold molecule formation These systems belong to the class of molecules commonlyreferred to as polar molecules characterized by a permanent electric dipole moment whichinduces an anisotropic long-range interaction between molecules Such a feature is predictedto reveal new effects in quantum degenerate gases [32] to be a building block of a molecularquantum information devices [33] or to design new setups for molecular optics [34] Heavymolecules with large permanent electric dipole moment are also considered as good candidates

Introductory Review

for the quest of the dipole moment of the electron [35 36 37] which represents a stringenttest of the standard model An overview of recent developements on cold polar moleculescan be found in [38] As discussed in [17 39] the short range Rminus6 variation of the atomndashatom interaction in the excited molecular states weakens photoassociation of mixed alkali pairscompared to the homonuclear ones On the other hand Azizi et al [17] demonstrated that thedecay back to the ground state or to the lowest triplet state is enhanced as the correspondingmolecular potential curves also behave as Rminus6 The overall rates for ultracold moleculeformation from mixed pairs are predicted to be only 10 to 50 times smaller than for homonuclearpairs This has been confirmed by the results obtained with RbCs by Kerman et al [40] and withKRb by Wang et al [41] In both cases the vibrational population of the ultracold moleculeshas been understood [42 43] and the observation of RbCs molecules in v = 0 has even beenreported [42]

The paper by WANG et al uses the advantage that ultracold molecules are produced in wellidentified vibrational levels with only few rotational levels populated by the photoassociationand spontaneous-decay sequence This simplifies the spectroscopy which otherwise would bevery complex because of the absence of inversion symmetry (ie the absence of a selectionrule associated with the gerade or ungerade character) in heteronuclear dimers more molecularstates are expected to interact together leading to complicated molecular level structure ThusWANG et al were able to analyse previously unknown molecular states of KRb confirmingtheoretical predictions and opening ways for new molecular state preparation

The work of HAIMBERGER et al from Rochester adds one more species of ultracoldmolecules NaCs following their preliminary study [12] In contrast with RbCs and KRbthe NaCs molecule has a large permanent dipole moment provided one can prepare lowvibrational levels of the ground state for further studies The group in Freiburg (KRAFT etal) also reports on their very recent observation of ultracold LiCs molecules which have thelargest dipole moment of all known alkali pairs This study is only at its beginning in contrastwith the other papers the PA step is not provided by a separate laser but by the trapping laserjust as Mancini et al observed ultracold KRb molecules [13] for the first time

It is worth noting that these successes are also built on accurate knowledge of the structureof these molecules brought by conventional high resolution spectroscopy over many yearsThe group at Hannover is a well-recognized specialist for this technique designed to producedata relevant for ultracold collisions ie the knowledge of levels close to the dissociation limitof the ground state in order to deduce scattering properties of the corresponding atom pairThis is illustrated as a nice complement to the work by HAIMBERGER et al with the paper onthe pair Na + Cs from DOCENKO et al the coupling between the singlet ground and the tripletmetastable state due to hyperfine interaction is fully modelled from a large data set of observedfluorescence spectra and predictions of Feshbach resonance are presented

Two theoretical papers explore new species and possible new mechanisms for ultracoldmolecule production AYMAR et al at CNRS-Orsay investigate the possibility of formingmolecules composed with francium and Rb or Cs motivated by the availability of cold franciumtraps which will be mixed in the near future with Rb or Cs traps [44 45] Using new quantumchemistry calculations of francium for computing potential curves they point out that theunusually large fine structure of francium plays an important role in the question of existingdouble well potentials which were helpful in producing ultracold Cs2 for the first time [24]

JUARROS et al at the Centre for Astrophysics at Harvard are interested in the formation of oneof the favourite molecules of astrophysicists namely LiH opening in a theoretical feasibilitystudy a different class of molecules Assuming sufficiently dense ultracold ensembles of Liand H they give quantitative results for molecular ground state population via the B1 statewhich seems to be preferable compared to the A1+ state

Introductory Review

Time allows fine cooking processes under optimal control

The photoassociation route described above relies on cw lasers for the excitation step and onspontaneous emission for the stabilization step While producing large amounts of ultracoldmolecules the main drawback of this sequence is that ultracold molecules are generally createdin a broad distribution of vibrational levels most often well above the v = 0 level Inthis respect the situation described by VATASESCU et al is probably an exceptional case Analternative has been explored through a two-colour stimulated Raman scheme [46 47 48] butthe efficiency of the process is severely limited by the reversibility of the process ie theultracold molecules can be dissociated through the same scheme

The idea of using laser pulses to overcome this limitation rose some time ago in thecontext of cold atom photoassociation with the model proposed by Machholm et al [49]based on a pumpndashdump scheme It is easy to realize that the characteristics of the pulses egintensity duration time delay offer control parameters to optimize the process efficiency verymuch along the ideas of adaptive optimal control initiated by Judson and Rabitz [50] Morerecently chirped laser pulses have been considered theoretically for optimizing the formationof ultracold molecules in a pumpndashdump scheme [51 52] None of these possibilities have beenexperimentally demonstrated yet even if a certain degree of control of cold collisions [53] andof dissociation of ultracold Rb2 molecules with chirped laser pulses [54] has been observed aswell as of ultracold Rb2 dissociation with feedback control [55]

Four theoretical papers in this issue intend to model possible experiments with Rb2 dimersas this is the species chosen by the experimental groups above The paper by KOCH et alassesses the feasibility of a formation scheme with picoseconds chirped lasers by calculatingthe number of created molecules per pulse with three different models for taking in account theinitial velocity distribution of the atoms POSCHINGER et al and BROWN AND WALMSLEY explorecomplementary ways to design shaped femtosecond pulses the former uses adaptive feedbackcontrol of the pulse amplitude and phase with a genetic algorithm while the latter focuses onintuitive pulse designs which could provide a starting point for a future experiment FinallySCHAFER-BUNG et al assume that the PA step is achieved via cw lasers creating an excited Rb2

molecule in a well-defined vibrational level and concentrate on the stabilization step througha pulse sequence designed by an optimal control procedure

In the absence of experiments approaching the proposed models above it is too early tocomment on the quality of these models However the theoretical investigations undoubtlyinclude more and more refinements which will certainly be very useful for guiding upcomingexperiments

lsquoRefrigeratingrsquo molecules

The laser schemes are very efficient in cooling but are not generally applicable at least formolecules For atoms the limits mainly come from the possible unavailability of laser sourcesfor exciting a good atomic transition But for molecules the wide spread of the spontaneousdecay into different quantum states limits the process to a few cooling cycles before themolecule is lost in a state which is not in resonance with the applied field even if one overlapsseveral light fields Chirped lasers are still to be explored for this application

As in solid state physics several cooling steps could progress to the desired goal ofultracold ensembles As a first step buffer-gas cooling is an idea which was successfullyapplied for the molecule CaH by the Doyle group at Harvard [56] which because of itsparamagnetic ground state could be magnetically trapped at temperatures around 100 mKOther molecules could be cooled with this method but not trapped so far In their paper

Introductory Review

BAKKER et al from the Berlin group of A Peters report a new setup for buffer-gas cooling andcould successfully demonstrate the cooling and trapping of Cr atoms The paper discusses indetail the necessary conditions for trapping paramagnetic molecules after buffer-gas coolingand proposes as good candidates CrH and MnH with their high spin ground states and not toolarge spinndashspin interactions

A very different cooling device for molecules is the helium nanodroplet expanding out ofa nozzle which equilibrates around a few tenths of a Kelvin The beam of droplets consistingof thousands of He atoms travels through a pickup cell where the atoms and molecules areloaded onto the droplet which will equilibrate again by vaporizing He atoms Under thesecircumstances metastable molecular states are formed as is demonstrated for alkali dimersALLARD et al from Graz report on new experiments on Rb2 and KRb where they observethe lowest triplet state of desorbing molecules from the droplet and additionally get the firstinformation about the distribution of populated molecular levels from the desorption Theaccompanying paper by BEUC et al from Zagreb gives valuable predictions for the expectedband profiles of absorption spectra from the metastable triplet state of KRb under the conditionsof such cold He droplets and will guide further experiments such as those from Graz The Hedroplets are a quite universal cold laboratory for a large manifold of molecules The groupin Rostock (see the paper by PRZYSTAWIK et al) applies it for silver dimers and reports on thebuilding of the metastable triplet state of Ag2 which is very interesting because Ag2 is locatedinside the He droplet whereas the alkali dimers sit on the surface of the droplet Thereforevery different interactions between the molecule and the droplet are expected This question isin view by the work of the group of F Stienkemeier at Freiburg (see the paper by CLAAS et al)where they studied K2 on the droplet and used the pumpndashprobe technique with femtosecondlaser pulses They find differences between the dynamics of photoionization in the gas phaseand on the droplet Also the timescale of the desorption process is identified

Supersonic expansion of gases other than He can lead to beams cooled in the translationalvelocity spread and internal energy to temperatures below 1 K Such cold samples are studiedfor weakly bound systems like van der Waals molecules KOPERSKI AND FRY apply this techniquefor spectroscopic studies of some metal noble gas compounds and especially to Hg2 for whichspectroscopic data are valuable for photoassociation experiments of trapped Hg atoms

Refrigerating is not universal slowing molecules

Parallel to the intensive studies of laser cooling of atoms like alkalis alkaline-earth elements anda few others or to the application of a coolant to widen the scope of molecules researchers haveaimed at creating slow molecules starting from fast molecular beams The main achievementsconcern the Stark deceleration technique [57] and the selection of slow molecules out of thefast ones [58] using guiding electric quadrupole fields With such techniques polar moleculeslike ND3 OH or CH2CO were successfully decelerated or selected Let us note that the heavypolar molecule YbF has been slowed down using the Stark deceleration approach for the questof permanent electric dipole moment of the electron [59]

New advances are reported by VAN DER MEERAKKER et al at the FHI in Berlin deceleratingNH in its metastable state a1 then converted inside a magnetic trap to the ground state X3minusby which accumulation of several decelerated packages could be achieved Additionally theypropose a nice idea for compressing the phase space by dumping to the ground state witha cw laser instead of a pulsed laser naturally assumed for packages arriving periodically intime With light fields it is not very difficult to obtain huge electric field strength by focussingThus in the paper by FULTON et al from Edinburgh deceleration is studied with a periodicoptical lattice from pulsed laser fields for NO and benzene reducing the kinetic energy by

Introductory Review

as much as 75 of the incoming one This might open research to much wider classes ofmolecules The paper by JUNG et al brings applications of the decelerated SO2 molecule intoview They measured the electric dipole moment of an excited state which they propose to usein a photodissociation experiment to produce cold radicals such as SO and O and the electricdipole moment could allow tuning of the photodissociation and thus the kinetic energy of thefragments

New recipes

After the fast development of different cooling recipes we need recipes and concepts forapplications With ultracold atomic ensembles in optical lattices one is able to model differentvariants of the BosendashHubbard system eg the transition from the Mott-insulator to thesuperfluid phase and the creation of ultracold dipolar molecular gases will add an importantnew ingredient by the anisotropy of the long range interaction to the richness of possible phasetransitions

But ultracold molecules opened the presently emerging field of ultracold chemistry witheither quantum effects (superchemistry with degenerate gases) and control of interactions withexternal fields (laser or static magnetic and electric fields) being new recipes in chemistry Firstexperimental observations of ultracold atomndashmolecule reactions have been reported [60 61]studying the lifetime of Cs2 ensembles embedded in Cs ensembles More theoretical modelsare required with which good experimental systems can be proposed or guided in future Thetheoretical work by WECK AND BALAKRISHNAN gives a general introduction to the physicalsituation of atomndashmolecule reactions at threshold and emphasizes the importance of theappearance of resonances otherwise low reaction rates are normally expected In the paper byYANG et al the link between ultracold collisions of atoms and molecules for quenching and theexperiments of buffer gas cooling mentioned above is presented for the example of rotationalquenching of CO through collision with H He or H2 in a wide range of energies from 10 μKto about 10 K as needed in the different stages of cooling processes The paper by GONZALEZ-SANCHEZ et al reports on similar aspects on the even simpler system OHminus in its ground state1+ with He atoms in 1S0 to be as complete as possible in the theoretical modelling Allthese calculations give rate constants which are of a magnitude to be observable in upcomingexperiments ultracold chemistry is probably richer than is thought by many chemists

Ion crystals but not traditional salt of the kitchen

Ion reactions play an important role in chemistry and for the cold regime the kitchen withinthe cold interstellar clouds comes immediately into view as a highly diluted system Thustrapped molecular ions certainly belong to a representation of the status of research of coldmolecules and their applications The group at Dusseldorf demonstrates in the paper by ROTH

et al the general applicability of sympathetic cooling of reaction partners like noble gas ionsor N+

2 and O+2 by laser cooled Be+ ions to produce a large variety of new cold ion samples eg

H+3 N2H+ etc These may serve for high-precision measurements for fundamental physical

questions like the time variation of fundamental constants or for studies of reaction processesrelevant in interstellar chemistry

Because of the strong Coulomb interaction between ions they appear regularly as Coulombcrystals if sufficiently cooled which gives great advantages because the ions within a crystalare well localized and thus addressable for particle by particle interaction and single particledetection To be quantum state specific in such studies the preparation of the initial physical

Introductory Review

condition is of fundamental importance This problem is addressed by two papers of theDrewsen group at Arhus In the first one by VOGELIUS et al they theoretically study the case ofrotational cooling of a polar molecule (here an ion which has an electric dipole moment withreference to its centre of mass) by optical coupling of the collective modes of a two-ion systemAs an example they describe the situation of MgH+ which is easily sympathetically cooled byCa+ In a second paper by the same authors they explore the possibility for probabilistic statepreparation in single molecular ions by projection measurements

Conclusion

Cold molecules offer a fantastic opportunity to develop strongly interdisciplinary research Twodomains are presently in view condensed matter physics with strongly correlated particlesand chemistry and superchemistry with the clear appearance of quantum effects in reactionprocesses This special issue on lsquoCold Moleculesrsquo gives insight into present lsquocookingrsquo inthe different laboratories which still concentrates on developing new methods and schemesto eventually proceed to systems which are most appropriate for the physical and chemicalquestions and applications When research on cold molecules started many scientists thought itwill be an artificial side-field of short duration But looking back the applicability of ultracoldatomic ensembles to model ideal solid state physics was not obvious Similarly the possiblerichness of superchemistry is unexplored but calculations of some systems clearly indicate thatthere is great hope of exciting results To harvest the fruits is probably more difficult becauseof the large number of degrees of freedom which we have to address by elegant experimentaland theoretical ideas and techniques not to be lost in the overwhelming routes which naturein principle has at its disposal and selects by evolution during millions of years We have torecognize the finite human lifetime Thus a thorough study of the papers provided by excellentlaboratories will be the right way of developing ideas during the short time available to us Wethank the authors for their contributions

Acknowledgments

The authors of this editorial acknowledge support from the Research and training Networkof the European Commission lsquoCold Moleculesrsquo (Contract HPRN- CT-2002-00290) and ODfrom the European Science Foundation through the network lsquoCollisions in Atom trapsrsquo

References

[1] Heinzen D J Wynar R Drummond P D and Kheruntsyan K V 2000 Superchemistry Dynamics of coupledatomic and molecular bose-einstein condensates Phys Rev Lett 84 5029

[2] httpwwwlacu-psudfrcoldmolecules[3] httpphysicsopenacukcats[4] httpquantendynamikphysikuni-freiburgdeles houches 2006[5] Chu S 1998 Nobel lecture the manipulation of neutral particles Rev Mod Phys 70 685[6] Cohen-Tannoudji C N 1998 Nobel lecture manipulating atoms with photons Rev Mod Phys 70 707[7] Phillips W D 1998 Nobel lecture laser cooling and trapping of neutral atoms Rev Mod Phys 70 721[8] Cornell E A and Wieman C E 2002 Nobel lecture BosendashEinstein condensation in a dilute gas the first 70 years

and some recent experiments Rev Mod Phys 74 875[9] Ketterle W 2002 Nobel lecture when atoms behaves as waves BosendashEinstein condensation and the atom laser

Rev Mod Phys 74 1131[10] Weiner J Bagnato V S Zilio S C and Julienne P S 1999 Experiments and theory in cold and ultracold collisions

Rev Mod Phys 71 1

Introductory Review

[11] Schloder U Engler H Schunemann U Grimmand R and Weidemuller M 1999 Cold inelastic collisions betweenlithium and cesium in a two- species magneto-optical trap Eur Phys J D 7 331

[12] Shaffer J P Chalupczak W and Bigelow N P 1999 Highly-excited states of ultracold molecules photoassociativespectroscopy of Na2 Phys Rev Lett 83 3621

[13] Mancini M W Telles G D Caires A R L Bagnato V S and Marcassa L G 2004 Observation of ultracoldground-state heteronuclear molecules Phys Rev Lett 92 133203

[14] Thorsheim H R Weiner J and Julienne P S 1987 Laser-induced photoassociation of ultracold sodium atomsPhys Rev Lett 58 2420

[15] Lett P D Helmerson K Philips W D Ratliff L P RolstonS L and Wagshul M E 1993 Spectroscopy of Na2 byphotoassociation of laser-cooled Na Phys Rev Lett 71 2200

[16] Miller J D Cline R A and Heinzen D J 1993 Photoassociation spectrum of ultracold Rb atoms Phys Rev Lett71 2204

[17] Azizi S Aymar M and Dulieu O 2004 Prospects for the formation of ultracold ground state polar moleculesfrom mixed alkali atom pair Eur Phys J D 31 195

[18] Lett P D Julienne P S and Philips W D 1995 Photoassociative spectroscopy of laser-cooled atoms Annu RevPhys Chem 46 423

[19] Stwalley W C and Wang H 1999 Photoassociation of ultracold atoms a new spectroscopic technique J MolSpectrosc 195 194

[20] Jones K M Tiesinga E Lett P D and Julienne P S 2006 Ultracold photoassociation spectroscopy Long-rangemolecules and atomic scattering Rep Prog Phys 78 483

[21] Stwalley W C Uang Y H and Pichler G 1978 Pure long-range molecules Phys Rev Lett 41 1164[22] Bohn J L and Julienne P S 1996 Semianalytic treatment of two-color photoassociation spectroscopy and control

of cold atoms Phys Rev A 54 R4637[23] Bohn J L and Julienne P S 1999 Semianalytic theory of laser-assisted resonant cold collisions Phys Rev A 60

414[24] Fioretti A Comparat D Crubellier A Dulieu O Masnou-Seeuws F and Pillet P 1998 Formation of Cs2 cold

molecules through photoassociation Phys Rev Lett 80 4402[25] C Gabbanini A Fioretti A Lucchesini S Gozzini and M Mazzoni 2000 Cold rubidium molecules formed in a

magneto-optical trap Phys Rev Lett 84 2814[26] Dion C M Drag C Dulieu O Laburthe Tolra B Masnou-Seeuws F and Pillet P 2001 Resonant coupling in the

formation of ultracold ground state molecules via photoassociation Phys Rev Lett 86 2253[27] Fatemi F K Jones K M Lett P D and Tiesinga E 2002 Ultracold ground-state molecule production in sodium

Phys Rev A 66 053401[28] Nikolov A N Eyler E E Wang X T Li J Wang H Stwalley W C and Gould P L 1999 Observation of ultracold

ground state potassium molecules Phys Rev Lett 82 703[29] Nikolov A N Enscher J R Eyler E E Wang H Stwalley W C and Gould P L 2000 Efficient production of ground

state potassium molecules at sub-mK temperatures by two-step photoassociation Phys Rev Lett 84 246[30] Band Y B and Julienne P S 1995 Ultracold molecule production by laser-cooled atom photoassociation Phys

Rev A 51 R4317[31] Lozeille J Fioretti A Gabbanini C Huang Y Pechkis H K Wang D Gould P L Eyler E E Stwalley W C

Aymar M and Dulieu O 2006 Detection of cold Rb2 triplet state molecules by two-photon ionization EurPhys J D 39

[32] Damski B Santos L Tiemann E Lewenstein M Kotochigova S Julienne P and Zoller P 2003 Creation of adipolar superfluid in optical lattices Phys Rev Lett 90 110401

[33] DeMille D 2002 Quantum computation with trapped polar molecules Phys Rev Lett 88 067901[34] Kallush S Segev B and Cote R 2005 Evanescent-wave mirror for ultracold diatomic polar molecules Phys Rev

Lett 95 163005[35] Sandars P G H 1967 Measurability of th eproton electric dipole moment Phys Rev Lett 19 1396[36] Hudson J J Sauer B E Tarbutt M R and Hinds E A 2002 Measurement of the electron electric dipole moment

using YbF molecules Phys Rev Lett 89 023003[37] Kawall D Bay F Bickman S Jiang Y and DeMille D 2004 Precision ZeemanndashStark spectroscopy of the

metastable a(1)3σ + state of PbO Phys Rev Lett 92 133007[38] Doyle J Friedrich B Krems R V and Masnou-Seeuws F 2004 Special issue on ultracold polar molecules 2004

Eur Phys J D 31 149 and following papers[39] Wang H Gould P L and Stwalley W C 1998 Fine-structure predissociation of ultracold photoassociated39K2

molecules observed by fragmentation spectroscopy Phys Rev Lett 80 476[40] Kerman A J Sage J M Sainis S Bergeman T and DeMille D 2004 Production and state-selective detection of

ultracold rbcs molecules Phys Rev Lett 92 153001

Introductory Review

[41] Wang D Qi J Stone M F Nikolayeva O Wang H Hattaway B Gensemer S D Gould P L Eyler E E andStwalley W C 2004 Photoassociative production and trapping of ultracold krb molecules Phys Rev Lett 93243005

[42] Sage J M Sainis S Bergeman T and DeMille D 2005 Optical production of ultracold polar molecules PhysRev Lett 94 203001

[43] Wang D Qi J Stone M F Nikolayeva O Hattaway B Gensemer S D Wang H Zemke W T Gould P LEyler E E and Stwalley W C 2004 The photoassociative spectroscopy photoassociative molecule formationand trapping of ultracold 39K85Rb Eur Phys J D 31 165

[44] Orozco L 2006 private communication[45] Sprouse G D Fliller R P Grossman J S Orozco L A and Pearson M R 2002 Traps for neutral radioactive atoms

Nucl Phys A 701 597[46] Abraham E R I McAlexander W I Sackett C A and Hulet R G 1995 Spectroscopic determination of the s-wave

scattering length of lithium Phys Rev Lett 74 1315[47] Tsai C C Freeland R S Vogels J M Boesten H M J M Gardner J R Heinzen D J and Verhaar B J 1997

Two-color photoassociation spectroscopy of ground state Rb2 Phys Rev Lett 79 1245[48] Laburthe Tolra B Drag C and Pillet P 2001 Formation of cold cesium molecules through stimulated raman

photoassociation Phys Rev A 64 R61401[49] Machholm M Giusti-Suzor A and Mies F H 1994 Photoassociation of atoms in ultracold collisions probed

bywave-packet dynamics Phys Rev A 50 5025[50] Judson R and Rabitz H 1992 Teaching lasers to control molecules Phys Rev Lett 68 1500[51] Vala J Dulieu O Masnou-Seeuws F and Kosloff R 2001 Coherent control of cold molecule formation through

photoassociation using a chirped pulsed laser field Phys Rev A 63 013412[52] Koch C P Luc-Koenig E and Masnou-Seeuws F 2006 Making ultracold molecules in a two color pump-dump

photoassociation scheme using chirped pulses Phys Rev A 73 033408[53] Wright M J Gensemer S D Vala J Kosloff R and Gould P L 2005 Control of ultracold collisions with frequency-

chirped light Phys Rev Lett 95 063001[54] Brown B L Dicks A J and Walmsley I A 2006 Coherent control of ultracold molecule dynamics in a magneto-

optical trap by use of chirped femtosecond laser pulses Phys Rev Lett 96 173002[55] Salzmann W Poschinger U Wester R Weidemuller M Merli A Weber S M Sauer F Plewicki M Weise F

Esparza A M Woste L and Lindinger A 2006 Coherent control with shaped femtosecond laser pulses appliedto ultracold molecules Phys Rev A 73 023414

[56] Weinstein J D deCarvalho R Kim J Patterson D Friedrich B and Doyle J M 1998 Magnetic trapping of atomicchromium Phys Rev A 57 R3173

[57] Bethlem H L and Meijer G 2003 Production and application of translationally cold molecules Int Rev PhysChem 22 73

[58] Rangwala S A Junglen T Rieger T Pinkse P W H and Rempe G 2003 Contnuous source of translationally colddipolar molecules Phys Rev A 64 043406

[59] Tarbutt M R Bethlem H L Hudson J J Ryabov V L Ryzhov V A Sauer B E Meijer G and Hinds E A 2004Slowing heavy ground-state molecules using an alternating gradient decelerator Phys Rev Lett 92 173002

[60] Staanum P Kraft S D Lange J Wester R and Weidemuller M 2006 Experimental investigation of ultracoldatom-molecule collisions Phys Rev Lett 96 023201

[61] Zahzam N Vogt T Mudrich M Comparat D and Pillet P 2006 Atomndashmolecule collisions in an optically trappedgas Phys Rev Lett 96 023202

Introductory Review

lsquoWell-cookedrsquo ultracold molecules stabilization

Stable ultracold molecules are needed to work with them during long times typically ofthe order of the trapping lifetime The lsquophotoassociative panrsquo produces ultracold excitedmolecules which tend to decay radiatively into pairs of hot atoms escaping from the lsquopanrsquo(or more physically from the atom trap) This kind of heating process can be turned intoan internal cooling process by choosing the right ingredients in order to enhance the decayof photoassociated molecules into ultracold long-lived molecules In 1997 the Orsay groupobserved for the first time such molecules starting from ultracold caesium atoms [24] thanksto both the peculiar properties of caesium dimers and to the detection method

The caesium dimer is one of two known molecules (the rubidium dimer is the other)whose purely long-range potential well reaches distances which are short enough to allow anefficient decay towards strongly bound levels of the lowest electronic states More preciselythe amplitude of the photoassociated wave function has to have significant values both at largedistances for the PA step and at short distance for the decay step These molecules are thendetected by two-photon resonant ionization yielding molecular ions which are easily collectedwith time-of-flight mass spectrometry As expected the same result was obtained shortly afterwith ultracold rubidium atoms [25] Another formation path through photoassociation is basedon the radial interaction between two electronic states and has been probed by Dion et al [26]on caesium PA This so-called lsquoresonant couplingrsquo relies on non-BornndashOppenheimer effectswhich are known to be present in almost all diatomic molecules and which are the cause forthe perturbations discussed in the papers by BERGEMAN et al and PICHLER et al in this issue Forthe lighter alkali dimers the task is not so easy as the long-range wells are located at too largedistances or the level densities do not favour resonant coupling Fatemi et al [27] from NISTwere however successful in observing with one-photon PA ultracold Na2 molecules in theirhighest vibrational levels much in the way which was originally proposed by Thorsheim et al[14] at the very end of their pioneering paper Nikolov et al from the University of Connecticut[28] were also able to detect a few ultracold K2 molecules exploiting the detailed knowledgeof the K2 structure supplied by conventional high resolution spectroscopy Shortly after thesame group demonstrated a more efficient formation scheme relying on two-step PA of coldpotassium atoms [29] theoretically worked out by Band and Julienne [30]

The new paper by HUANG et al from the University of Connecticut shows that the PA stepand the detection step are now so well mastered that they give access to the spectroscopy ofthe ultracold Rb2 molecules created in their absolute ground state X1+

g This is particularlyrelevant for optimizing the PA transitions which would favour the formation of ultracoldmolecules in low vibrational levels and ultimately in the lowest v = 0 level With a similargoal VATASESCU et al investigate the tunnelling effect in a double well potential of Cs2 whichis thought to be responsible for anomalous features in the PA spectrum of ultracold caesiumand predicted efficient production of ultracold Cs2 molecules in a fairly narrow distribution oflow vibrational levels (around v = 7) of the metastable a3+

u Let us note that HUANG et al(as well as the similar study of [31]) also demonstrate that the spectroscopy of the molecularstates involved in the first step of the two-photon resonant detection can be precisely assigned

The heteronuclear alkali diatomic molecules appear naturally as further candidates forPA and cold molecule formation These systems belong to the class of molecules commonlyreferred to as polar molecules characterized by a permanent electric dipole moment whichinduces an anisotropic long-range interaction between molecules Such a feature is predictedto reveal new effects in quantum degenerate gases [32] to be a building block of a molecularquantum information devices [33] or to design new setups for molecular optics [34] Heavymolecules with large permanent electric dipole moment are also considered as good candidates

Introductory Review

for the quest of the dipole moment of the electron [35 36 37] which represents a stringenttest of the standard model An overview of recent developements on cold polar moleculescan be found in [38] As discussed in [17 39] the short range Rminus6 variation of the atomndashatom interaction in the excited molecular states weakens photoassociation of mixed alkali pairscompared to the homonuclear ones On the other hand Azizi et al [17] demonstrated that thedecay back to the ground state or to the lowest triplet state is enhanced as the correspondingmolecular potential curves also behave as Rminus6 The overall rates for ultracold moleculeformation from mixed pairs are predicted to be only 10 to 50 times smaller than for homonuclearpairs This has been confirmed by the results obtained with RbCs by Kerman et al [40] and withKRb by Wang et al [41] In both cases the vibrational population of the ultracold moleculeshas been understood [42 43] and the observation of RbCs molecules in v = 0 has even beenreported [42]

The paper by WANG et al uses the advantage that ultracold molecules are produced in wellidentified vibrational levels with only few rotational levels populated by the photoassociationand spontaneous-decay sequence This simplifies the spectroscopy which otherwise would bevery complex because of the absence of inversion symmetry (ie the absence of a selectionrule associated with the gerade or ungerade character) in heteronuclear dimers more molecularstates are expected to interact together leading to complicated molecular level structure ThusWANG et al were able to analyse previously unknown molecular states of KRb confirmingtheoretical predictions and opening ways for new molecular state preparation

The work of HAIMBERGER et al from Rochester adds one more species of ultracoldmolecules NaCs following their preliminary study [12] In contrast with RbCs and KRbthe NaCs molecule has a large permanent dipole moment provided one can prepare lowvibrational levels of the ground state for further studies The group in Freiburg (KRAFT etal) also reports on their very recent observation of ultracold LiCs molecules which have thelargest dipole moment of all known alkali pairs This study is only at its beginning in contrastwith the other papers the PA step is not provided by a separate laser but by the trapping laserjust as Mancini et al observed ultracold KRb molecules [13] for the first time

It is worth noting that these successes are also built on accurate knowledge of the structureof these molecules brought by conventional high resolution spectroscopy over many yearsThe group at Hannover is a well-recognized specialist for this technique designed to producedata relevant for ultracold collisions ie the knowledge of levels close to the dissociation limitof the ground state in order to deduce scattering properties of the corresponding atom pairThis is illustrated as a nice complement to the work by HAIMBERGER et al with the paper onthe pair Na + Cs from DOCENKO et al the coupling between the singlet ground and the tripletmetastable state due to hyperfine interaction is fully modelled from a large data set of observedfluorescence spectra and predictions of Feshbach resonance are presented

Two theoretical papers explore new species and possible new mechanisms for ultracoldmolecule production AYMAR et al at CNRS-Orsay investigate the possibility of formingmolecules composed with francium and Rb or Cs motivated by the availability of cold franciumtraps which will be mixed in the near future with Rb or Cs traps [44 45] Using new quantumchemistry calculations of francium for computing potential curves they point out that theunusually large fine structure of francium plays an important role in the question of existingdouble well potentials which were helpful in producing ultracold Cs2 for the first time [24]

JUARROS et al at the Centre for Astrophysics at Harvard are interested in the formation of oneof the favourite molecules of astrophysicists namely LiH opening in a theoretical feasibilitystudy a different class of molecules Assuming sufficiently dense ultracold ensembles of Liand H they give quantitative results for molecular ground state population via the B1 statewhich seems to be preferable compared to the A1+ state

Introductory Review

Time allows fine cooking processes under optimal control

The photoassociation route described above relies on cw lasers for the excitation step and onspontaneous emission for the stabilization step While producing large amounts of ultracoldmolecules the main drawback of this sequence is that ultracold molecules are generally createdin a broad distribution of vibrational levels most often well above the v = 0 level Inthis respect the situation described by VATASESCU et al is probably an exceptional case Analternative has been explored through a two-colour stimulated Raman scheme [46 47 48] butthe efficiency of the process is severely limited by the reversibility of the process ie theultracold molecules can be dissociated through the same scheme

The idea of using laser pulses to overcome this limitation rose some time ago in thecontext of cold atom photoassociation with the model proposed by Machholm et al [49]based on a pumpndashdump scheme It is easy to realize that the characteristics of the pulses egintensity duration time delay offer control parameters to optimize the process efficiency verymuch along the ideas of adaptive optimal control initiated by Judson and Rabitz [50] Morerecently chirped laser pulses have been considered theoretically for optimizing the formationof ultracold molecules in a pumpndashdump scheme [51 52] None of these possibilities have beenexperimentally demonstrated yet even if a certain degree of control of cold collisions [53] andof dissociation of ultracold Rb2 molecules with chirped laser pulses [54] has been observed aswell as of ultracold Rb2 dissociation with feedback control [55]

Four theoretical papers in this issue intend to model possible experiments with Rb2 dimersas this is the species chosen by the experimental groups above The paper by KOCH et alassesses the feasibility of a formation scheme with picoseconds chirped lasers by calculatingthe number of created molecules per pulse with three different models for taking in account theinitial velocity distribution of the atoms POSCHINGER et al and BROWN AND WALMSLEY explorecomplementary ways to design shaped femtosecond pulses the former uses adaptive feedbackcontrol of the pulse amplitude and phase with a genetic algorithm while the latter focuses onintuitive pulse designs which could provide a starting point for a future experiment FinallySCHAFER-BUNG et al assume that the PA step is achieved via cw lasers creating an excited Rb2

molecule in a well-defined vibrational level and concentrate on the stabilization step througha pulse sequence designed by an optimal control procedure

In the absence of experiments approaching the proposed models above it is too early tocomment on the quality of these models However the theoretical investigations undoubtlyinclude more and more refinements which will certainly be very useful for guiding upcomingexperiments

lsquoRefrigeratingrsquo molecules

The laser schemes are very efficient in cooling but are not generally applicable at least formolecules For atoms the limits mainly come from the possible unavailability of laser sourcesfor exciting a good atomic transition But for molecules the wide spread of the spontaneousdecay into different quantum states limits the process to a few cooling cycles before themolecule is lost in a state which is not in resonance with the applied field even if one overlapsseveral light fields Chirped lasers are still to be explored for this application

As in solid state physics several cooling steps could progress to the desired goal ofultracold ensembles As a first step buffer-gas cooling is an idea which was successfullyapplied for the molecule CaH by the Doyle group at Harvard [56] which because of itsparamagnetic ground state could be magnetically trapped at temperatures around 100 mKOther molecules could be cooled with this method but not trapped so far In their paper

Introductory Review

BAKKER et al from the Berlin group of A Peters report a new setup for buffer-gas cooling andcould successfully demonstrate the cooling and trapping of Cr atoms The paper discusses indetail the necessary conditions for trapping paramagnetic molecules after buffer-gas coolingand proposes as good candidates CrH and MnH with their high spin ground states and not toolarge spinndashspin interactions

A very different cooling device for molecules is the helium nanodroplet expanding out ofa nozzle which equilibrates around a few tenths of a Kelvin The beam of droplets consistingof thousands of He atoms travels through a pickup cell where the atoms and molecules areloaded onto the droplet which will equilibrate again by vaporizing He atoms Under thesecircumstances metastable molecular states are formed as is demonstrated for alkali dimersALLARD et al from Graz report on new experiments on Rb2 and KRb where they observethe lowest triplet state of desorbing molecules from the droplet and additionally get the firstinformation about the distribution of populated molecular levels from the desorption Theaccompanying paper by BEUC et al from Zagreb gives valuable predictions for the expectedband profiles of absorption spectra from the metastable triplet state of KRb under the conditionsof such cold He droplets and will guide further experiments such as those from Graz The Hedroplets are a quite universal cold laboratory for a large manifold of molecules The groupin Rostock (see the paper by PRZYSTAWIK et al) applies it for silver dimers and reports on thebuilding of the metastable triplet state of Ag2 which is very interesting because Ag2 is locatedinside the He droplet whereas the alkali dimers sit on the surface of the droplet Thereforevery different interactions between the molecule and the droplet are expected This question isin view by the work of the group of F Stienkemeier at Freiburg (see the paper by CLAAS et al)where they studied K2 on the droplet and used the pumpndashprobe technique with femtosecondlaser pulses They find differences between the dynamics of photoionization in the gas phaseand on the droplet Also the timescale of the desorption process is identified

Supersonic expansion of gases other than He can lead to beams cooled in the translationalvelocity spread and internal energy to temperatures below 1 K Such cold samples are studiedfor weakly bound systems like van der Waals molecules KOPERSKI AND FRY apply this techniquefor spectroscopic studies of some metal noble gas compounds and especially to Hg2 for whichspectroscopic data are valuable for photoassociation experiments of trapped Hg atoms

Refrigerating is not universal slowing molecules

Parallel to the intensive studies of laser cooling of atoms like alkalis alkaline-earth elements anda few others or to the application of a coolant to widen the scope of molecules researchers haveaimed at creating slow molecules starting from fast molecular beams The main achievementsconcern the Stark deceleration technique [57] and the selection of slow molecules out of thefast ones [58] using guiding electric quadrupole fields With such techniques polar moleculeslike ND3 OH or CH2CO were successfully decelerated or selected Let us note that the heavypolar molecule YbF has been slowed down using the Stark deceleration approach for the questof permanent electric dipole moment of the electron [59]

New advances are reported by VAN DER MEERAKKER et al at the FHI in Berlin deceleratingNH in its metastable state a1 then converted inside a magnetic trap to the ground state X3minusby which accumulation of several decelerated packages could be achieved Additionally theypropose a nice idea for compressing the phase space by dumping to the ground state witha cw laser instead of a pulsed laser naturally assumed for packages arriving periodically intime With light fields it is not very difficult to obtain huge electric field strength by focussingThus in the paper by FULTON et al from Edinburgh deceleration is studied with a periodicoptical lattice from pulsed laser fields for NO and benzene reducing the kinetic energy by

Introductory Review

as much as 75 of the incoming one This might open research to much wider classes ofmolecules The paper by JUNG et al brings applications of the decelerated SO2 molecule intoview They measured the electric dipole moment of an excited state which they propose to usein a photodissociation experiment to produce cold radicals such as SO and O and the electricdipole moment could allow tuning of the photodissociation and thus the kinetic energy of thefragments

New recipes

After the fast development of different cooling recipes we need recipes and concepts forapplications With ultracold atomic ensembles in optical lattices one is able to model differentvariants of the BosendashHubbard system eg the transition from the Mott-insulator to thesuperfluid phase and the creation of ultracold dipolar molecular gases will add an importantnew ingredient by the anisotropy of the long range interaction to the richness of possible phasetransitions

But ultracold molecules opened the presently emerging field of ultracold chemistry witheither quantum effects (superchemistry with degenerate gases) and control of interactions withexternal fields (laser or static magnetic and electric fields) being new recipes in chemistry Firstexperimental observations of ultracold atomndashmolecule reactions have been reported [60 61]studying the lifetime of Cs2 ensembles embedded in Cs ensembles More theoretical modelsare required with which good experimental systems can be proposed or guided in future Thetheoretical work by WECK AND BALAKRISHNAN gives a general introduction to the physicalsituation of atomndashmolecule reactions at threshold and emphasizes the importance of theappearance of resonances otherwise low reaction rates are normally expected In the paper byYANG et al the link between ultracold collisions of atoms and molecules for quenching and theexperiments of buffer gas cooling mentioned above is presented for the example of rotationalquenching of CO through collision with H He or H2 in a wide range of energies from 10 μKto about 10 K as needed in the different stages of cooling processes The paper by GONZALEZ-SANCHEZ et al reports on similar aspects on the even simpler system OHminus in its ground state1+ with He atoms in 1S0 to be as complete as possible in the theoretical modelling Allthese calculations give rate constants which are of a magnitude to be observable in upcomingexperiments ultracold chemistry is probably richer than is thought by many chemists

Ion crystals but not traditional salt of the kitchen

Ion reactions play an important role in chemistry and for the cold regime the kitchen withinthe cold interstellar clouds comes immediately into view as a highly diluted system Thustrapped molecular ions certainly belong to a representation of the status of research of coldmolecules and their applications The group at Dusseldorf demonstrates in the paper by ROTH

et al the general applicability of sympathetic cooling of reaction partners like noble gas ionsor N+

2 and O+2 by laser cooled Be+ ions to produce a large variety of new cold ion samples eg

H+3 N2H+ etc These may serve for high-precision measurements for fundamental physical

questions like the time variation of fundamental constants or for studies of reaction processesrelevant in interstellar chemistry

Because of the strong Coulomb interaction between ions they appear regularly as Coulombcrystals if sufficiently cooled which gives great advantages because the ions within a crystalare well localized and thus addressable for particle by particle interaction and single particledetection To be quantum state specific in such studies the preparation of the initial physical

Introductory Review

condition is of fundamental importance This problem is addressed by two papers of theDrewsen group at Arhus In the first one by VOGELIUS et al they theoretically study the case ofrotational cooling of a polar molecule (here an ion which has an electric dipole moment withreference to its centre of mass) by optical coupling of the collective modes of a two-ion systemAs an example they describe the situation of MgH+ which is easily sympathetically cooled byCa+ In a second paper by the same authors they explore the possibility for probabilistic statepreparation in single molecular ions by projection measurements

Conclusion

Cold molecules offer a fantastic opportunity to develop strongly interdisciplinary research Twodomains are presently in view condensed matter physics with strongly correlated particlesand chemistry and superchemistry with the clear appearance of quantum effects in reactionprocesses This special issue on lsquoCold Moleculesrsquo gives insight into present lsquocookingrsquo inthe different laboratories which still concentrates on developing new methods and schemesto eventually proceed to systems which are most appropriate for the physical and chemicalquestions and applications When research on cold molecules started many scientists thought itwill be an artificial side-field of short duration But looking back the applicability of ultracoldatomic ensembles to model ideal solid state physics was not obvious Similarly the possiblerichness of superchemistry is unexplored but calculations of some systems clearly indicate thatthere is great hope of exciting results To harvest the fruits is probably more difficult becauseof the large number of degrees of freedom which we have to address by elegant experimentaland theoretical ideas and techniques not to be lost in the overwhelming routes which naturein principle has at its disposal and selects by evolution during millions of years We have torecognize the finite human lifetime Thus a thorough study of the papers provided by excellentlaboratories will be the right way of developing ideas during the short time available to us Wethank the authors for their contributions

Acknowledgments

The authors of this editorial acknowledge support from the Research and training Networkof the European Commission lsquoCold Moleculesrsquo (Contract HPRN- CT-2002-00290) and ODfrom the European Science Foundation through the network lsquoCollisions in Atom trapsrsquo

References

[1] Heinzen D J Wynar R Drummond P D and Kheruntsyan K V 2000 Superchemistry Dynamics of coupledatomic and molecular bose-einstein condensates Phys Rev Lett 84 5029

[2] httpwwwlacu-psudfrcoldmolecules[3] httpphysicsopenacukcats[4] httpquantendynamikphysikuni-freiburgdeles houches 2006[5] Chu S 1998 Nobel lecture the manipulation of neutral particles Rev Mod Phys 70 685[6] Cohen-Tannoudji C N 1998 Nobel lecture manipulating atoms with photons Rev Mod Phys 70 707[7] Phillips W D 1998 Nobel lecture laser cooling and trapping of neutral atoms Rev Mod Phys 70 721[8] Cornell E A and Wieman C E 2002 Nobel lecture BosendashEinstein condensation in a dilute gas the first 70 years

and some recent experiments Rev Mod Phys 74 875[9] Ketterle W 2002 Nobel lecture when atoms behaves as waves BosendashEinstein condensation and the atom laser

Rev Mod Phys 74 1131[10] Weiner J Bagnato V S Zilio S C and Julienne P S 1999 Experiments and theory in cold and ultracold collisions

Rev Mod Phys 71 1

Introductory Review

[11] Schloder U Engler H Schunemann U Grimmand R and Weidemuller M 1999 Cold inelastic collisions betweenlithium and cesium in a two- species magneto-optical trap Eur Phys J D 7 331

[12] Shaffer J P Chalupczak W and Bigelow N P 1999 Highly-excited states of ultracold molecules photoassociativespectroscopy of Na2 Phys Rev Lett 83 3621

[13] Mancini M W Telles G D Caires A R L Bagnato V S and Marcassa L G 2004 Observation of ultracoldground-state heteronuclear molecules Phys Rev Lett 92 133203

[14] Thorsheim H R Weiner J and Julienne P S 1987 Laser-induced photoassociation of ultracold sodium atomsPhys Rev Lett 58 2420

[15] Lett P D Helmerson K Philips W D Ratliff L P RolstonS L and Wagshul M E 1993 Spectroscopy of Na2 byphotoassociation of laser-cooled Na Phys Rev Lett 71 2200

[16] Miller J D Cline R A and Heinzen D J 1993 Photoassociation spectrum of ultracold Rb atoms Phys Rev Lett71 2204

[17] Azizi S Aymar M and Dulieu O 2004 Prospects for the formation of ultracold ground state polar moleculesfrom mixed alkali atom pair Eur Phys J D 31 195

[18] Lett P D Julienne P S and Philips W D 1995 Photoassociative spectroscopy of laser-cooled atoms Annu RevPhys Chem 46 423

[19] Stwalley W C and Wang H 1999 Photoassociation of ultracold atoms a new spectroscopic technique J MolSpectrosc 195 194

[20] Jones K M Tiesinga E Lett P D and Julienne P S 2006 Ultracold photoassociation spectroscopy Long-rangemolecules and atomic scattering Rep Prog Phys 78 483

[21] Stwalley W C Uang Y H and Pichler G 1978 Pure long-range molecules Phys Rev Lett 41 1164[22] Bohn J L and Julienne P S 1996 Semianalytic treatment of two-color photoassociation spectroscopy and control

of cold atoms Phys Rev A 54 R4637[23] Bohn J L and Julienne P S 1999 Semianalytic theory of laser-assisted resonant cold collisions Phys Rev A 60

414[24] Fioretti A Comparat D Crubellier A Dulieu O Masnou-Seeuws F and Pillet P 1998 Formation of Cs2 cold

molecules through photoassociation Phys Rev Lett 80 4402[25] C Gabbanini A Fioretti A Lucchesini S Gozzini and M Mazzoni 2000 Cold rubidium molecules formed in a

magneto-optical trap Phys Rev Lett 84 2814[26] Dion C M Drag C Dulieu O Laburthe Tolra B Masnou-Seeuws F and Pillet P 2001 Resonant coupling in the

formation of ultracold ground state molecules via photoassociation Phys Rev Lett 86 2253[27] Fatemi F K Jones K M Lett P D and Tiesinga E 2002 Ultracold ground-state molecule production in sodium

Phys Rev A 66 053401[28] Nikolov A N Eyler E E Wang X T Li J Wang H Stwalley W C and Gould P L 1999 Observation of ultracold

ground state potassium molecules Phys Rev Lett 82 703[29] Nikolov A N Enscher J R Eyler E E Wang H Stwalley W C and Gould P L 2000 Efficient production of ground

state potassium molecules at sub-mK temperatures by two-step photoassociation Phys Rev Lett 84 246[30] Band Y B and Julienne P S 1995 Ultracold molecule production by laser-cooled atom photoassociation Phys

Rev A 51 R4317[31] Lozeille J Fioretti A Gabbanini C Huang Y Pechkis H K Wang D Gould P L Eyler E E Stwalley W C

Aymar M and Dulieu O 2006 Detection of cold Rb2 triplet state molecules by two-photon ionization EurPhys J D 39

[32] Damski B Santos L Tiemann E Lewenstein M Kotochigova S Julienne P and Zoller P 2003 Creation of adipolar superfluid in optical lattices Phys Rev Lett 90 110401

[33] DeMille D 2002 Quantum computation with trapped polar molecules Phys Rev Lett 88 067901[34] Kallush S Segev B and Cote R 2005 Evanescent-wave mirror for ultracold diatomic polar molecules Phys Rev

Lett 95 163005[35] Sandars P G H 1967 Measurability of th eproton electric dipole moment Phys Rev Lett 19 1396[36] Hudson J J Sauer B E Tarbutt M R and Hinds E A 2002 Measurement of the electron electric dipole moment

using YbF molecules Phys Rev Lett 89 023003[37] Kawall D Bay F Bickman S Jiang Y and DeMille D 2004 Precision ZeemanndashStark spectroscopy of the

metastable a(1)3σ + state of PbO Phys Rev Lett 92 133007[38] Doyle J Friedrich B Krems R V and Masnou-Seeuws F 2004 Special issue on ultracold polar molecules 2004

Eur Phys J D 31 149 and following papers[39] Wang H Gould P L and Stwalley W C 1998 Fine-structure predissociation of ultracold photoassociated39K2

molecules observed by fragmentation spectroscopy Phys Rev Lett 80 476[40] Kerman A J Sage J M Sainis S Bergeman T and DeMille D 2004 Production and state-selective detection of

ultracold rbcs molecules Phys Rev Lett 92 153001

Introductory Review

[41] Wang D Qi J Stone M F Nikolayeva O Wang H Hattaway B Gensemer S D Gould P L Eyler E E andStwalley W C 2004 Photoassociative production and trapping of ultracold krb molecules Phys Rev Lett 93243005

[42] Sage J M Sainis S Bergeman T and DeMille D 2005 Optical production of ultracold polar molecules PhysRev Lett 94 203001

[43] Wang D Qi J Stone M F Nikolayeva O Hattaway B Gensemer S D Wang H Zemke W T Gould P LEyler E E and Stwalley W C 2004 The photoassociative spectroscopy photoassociative molecule formationand trapping of ultracold 39K85Rb Eur Phys J D 31 165

[44] Orozco L 2006 private communication[45] Sprouse G D Fliller R P Grossman J S Orozco L A and Pearson M R 2002 Traps for neutral radioactive atoms

Nucl Phys A 701 597[46] Abraham E R I McAlexander W I Sackett C A and Hulet R G 1995 Spectroscopic determination of the s-wave

scattering length of lithium Phys Rev Lett 74 1315[47] Tsai C C Freeland R S Vogels J M Boesten H M J M Gardner J R Heinzen D J and Verhaar B J 1997

Two-color photoassociation spectroscopy of ground state Rb2 Phys Rev Lett 79 1245[48] Laburthe Tolra B Drag C and Pillet P 2001 Formation of cold cesium molecules through stimulated raman

photoassociation Phys Rev A 64 R61401[49] Machholm M Giusti-Suzor A and Mies F H 1994 Photoassociation of atoms in ultracold collisions probed

bywave-packet dynamics Phys Rev A 50 5025[50] Judson R and Rabitz H 1992 Teaching lasers to control molecules Phys Rev Lett 68 1500[51] Vala J Dulieu O Masnou-Seeuws F and Kosloff R 2001 Coherent control of cold molecule formation through

photoassociation using a chirped pulsed laser field Phys Rev A 63 013412[52] Koch C P Luc-Koenig E and Masnou-Seeuws F 2006 Making ultracold molecules in a two color pump-dump

photoassociation scheme using chirped pulses Phys Rev A 73 033408[53] Wright M J Gensemer S D Vala J Kosloff R and Gould P L 2005 Control of ultracold collisions with frequency-

chirped light Phys Rev Lett 95 063001[54] Brown B L Dicks A J and Walmsley I A 2006 Coherent control of ultracold molecule dynamics in a magneto-

optical trap by use of chirped femtosecond laser pulses Phys Rev Lett 96 173002[55] Salzmann W Poschinger U Wester R Weidemuller M Merli A Weber S M Sauer F Plewicki M Weise F

Esparza A M Woste L and Lindinger A 2006 Coherent control with shaped femtosecond laser pulses appliedto ultracold molecules Phys Rev A 73 023414

[56] Weinstein J D deCarvalho R Kim J Patterson D Friedrich B and Doyle J M 1998 Magnetic trapping of atomicchromium Phys Rev A 57 R3173

[57] Bethlem H L and Meijer G 2003 Production and application of translationally cold molecules Int Rev PhysChem 22 73

[58] Rangwala S A Junglen T Rieger T Pinkse P W H and Rempe G 2003 Contnuous source of translationally colddipolar molecules Phys Rev A 64 043406

[59] Tarbutt M R Bethlem H L Hudson J J Ryabov V L Ryzhov V A Sauer B E Meijer G and Hinds E A 2004Slowing heavy ground-state molecules using an alternating gradient decelerator Phys Rev Lett 92 173002

[60] Staanum P Kraft S D Lange J Wester R and Weidemuller M 2006 Experimental investigation of ultracoldatom-molecule collisions Phys Rev Lett 96 023201

[61] Zahzam N Vogt T Mudrich M Comparat D and Pillet P 2006 Atomndashmolecule collisions in an optically trappedgas Phys Rev Lett 96 023202

Introductory Review

for the quest of the dipole moment of the electron [35 36 37] which represents a stringenttest of the standard model An overview of recent developements on cold polar moleculescan be found in [38] As discussed in [17 39] the short range Rminus6 variation of the atomndashatom interaction in the excited molecular states weakens photoassociation of mixed alkali pairscompared to the homonuclear ones On the other hand Azizi et al [17] demonstrated that thedecay back to the ground state or to the lowest triplet state is enhanced as the correspondingmolecular potential curves also behave as Rminus6 The overall rates for ultracold moleculeformation from mixed pairs are predicted to be only 10 to 50 times smaller than for homonuclearpairs This has been confirmed by the results obtained with RbCs by Kerman et al [40] and withKRb by Wang et al [41] In both cases the vibrational population of the ultracold moleculeshas been understood [42 43] and the observation of RbCs molecules in v = 0 has even beenreported [42]

The paper by WANG et al uses the advantage that ultracold molecules are produced in wellidentified vibrational levels with only few rotational levels populated by the photoassociationand spontaneous-decay sequence This simplifies the spectroscopy which otherwise would bevery complex because of the absence of inversion symmetry (ie the absence of a selectionrule associated with the gerade or ungerade character) in heteronuclear dimers more molecularstates are expected to interact together leading to complicated molecular level structure ThusWANG et al were able to analyse previously unknown molecular states of KRb confirmingtheoretical predictions and opening ways for new molecular state preparation

The work of HAIMBERGER et al from Rochester adds one more species of ultracoldmolecules NaCs following their preliminary study [12] In contrast with RbCs and KRbthe NaCs molecule has a large permanent dipole moment provided one can prepare lowvibrational levels of the ground state for further studies The group in Freiburg (KRAFT etal) also reports on their very recent observation of ultracold LiCs molecules which have thelargest dipole moment of all known alkali pairs This study is only at its beginning in contrastwith the other papers the PA step is not provided by a separate laser but by the trapping laserjust as Mancini et al observed ultracold KRb molecules [13] for the first time

It is worth noting that these successes are also built on accurate knowledge of the structureof these molecules brought by conventional high resolution spectroscopy over many yearsThe group at Hannover is a well-recognized specialist for this technique designed to producedata relevant for ultracold collisions ie the knowledge of levels close to the dissociation limitof the ground state in order to deduce scattering properties of the corresponding atom pairThis is illustrated as a nice complement to the work by HAIMBERGER et al with the paper onthe pair Na + Cs from DOCENKO et al the coupling between the singlet ground and the tripletmetastable state due to hyperfine interaction is fully modelled from a large data set of observedfluorescence spectra and predictions of Feshbach resonance are presented

Two theoretical papers explore new species and possible new mechanisms for ultracoldmolecule production AYMAR et al at CNRS-Orsay investigate the possibility of formingmolecules composed with francium and Rb or Cs motivated by the availability of cold franciumtraps which will be mixed in the near future with Rb or Cs traps [44 45] Using new quantumchemistry calculations of francium for computing potential curves they point out that theunusually large fine structure of francium plays an important role in the question of existingdouble well potentials which were helpful in producing ultracold Cs2 for the first time [24]

JUARROS et al at the Centre for Astrophysics at Harvard are interested in the formation of oneof the favourite molecules of astrophysicists namely LiH opening in a theoretical feasibilitystudy a different class of molecules Assuming sufficiently dense ultracold ensembles of Liand H they give quantitative results for molecular ground state population via the B1 statewhich seems to be preferable compared to the A1+ state

Introductory Review

Time allows fine cooking processes under optimal control

The photoassociation route described above relies on cw lasers for the excitation step and onspontaneous emission for the stabilization step While producing large amounts of ultracoldmolecules the main drawback of this sequence is that ultracold molecules are generally createdin a broad distribution of vibrational levels most often well above the v = 0 level Inthis respect the situation described by VATASESCU et al is probably an exceptional case Analternative has been explored through a two-colour stimulated Raman scheme [46 47 48] butthe efficiency of the process is severely limited by the reversibility of the process ie theultracold molecules can be dissociated through the same scheme

The idea of using laser pulses to overcome this limitation rose some time ago in thecontext of cold atom photoassociation with the model proposed by Machholm et al [49]based on a pumpndashdump scheme It is easy to realize that the characteristics of the pulses egintensity duration time delay offer control parameters to optimize the process efficiency verymuch along the ideas of adaptive optimal control initiated by Judson and Rabitz [50] Morerecently chirped laser pulses have been considered theoretically for optimizing the formationof ultracold molecules in a pumpndashdump scheme [51 52] None of these possibilities have beenexperimentally demonstrated yet even if a certain degree of control of cold collisions [53] andof dissociation of ultracold Rb2 molecules with chirped laser pulses [54] has been observed aswell as of ultracold Rb2 dissociation with feedback control [55]

Four theoretical papers in this issue intend to model possible experiments with Rb2 dimersas this is the species chosen by the experimental groups above The paper by KOCH et alassesses the feasibility of a formation scheme with picoseconds chirped lasers by calculatingthe number of created molecules per pulse with three different models for taking in account theinitial velocity distribution of the atoms POSCHINGER et al and BROWN AND WALMSLEY explorecomplementary ways to design shaped femtosecond pulses the former uses adaptive feedbackcontrol of the pulse amplitude and phase with a genetic algorithm while the latter focuses onintuitive pulse designs which could provide a starting point for a future experiment FinallySCHAFER-BUNG et al assume that the PA step is achieved via cw lasers creating an excited Rb2

molecule in a well-defined vibrational level and concentrate on the stabilization step througha pulse sequence designed by an optimal control procedure

In the absence of experiments approaching the proposed models above it is too early tocomment on the quality of these models However the theoretical investigations undoubtlyinclude more and more refinements which will certainly be very useful for guiding upcomingexperiments

lsquoRefrigeratingrsquo molecules

The laser schemes are very efficient in cooling but are not generally applicable at least formolecules For atoms the limits mainly come from the possible unavailability of laser sourcesfor exciting a good atomic transition But for molecules the wide spread of the spontaneousdecay into different quantum states limits the process to a few cooling cycles before themolecule is lost in a state which is not in resonance with the applied field even if one overlapsseveral light fields Chirped lasers are still to be explored for this application

As in solid state physics several cooling steps could progress to the desired goal ofultracold ensembles As a first step buffer-gas cooling is an idea which was successfullyapplied for the molecule CaH by the Doyle group at Harvard [56] which because of itsparamagnetic ground state could be magnetically trapped at temperatures around 100 mKOther molecules could be cooled with this method but not trapped so far In their paper

Introductory Review

BAKKER et al from the Berlin group of A Peters report a new setup for buffer-gas cooling andcould successfully demonstrate the cooling and trapping of Cr atoms The paper discusses indetail the necessary conditions for trapping paramagnetic molecules after buffer-gas coolingand proposes as good candidates CrH and MnH with their high spin ground states and not toolarge spinndashspin interactions

A very different cooling device for molecules is the helium nanodroplet expanding out ofa nozzle which equilibrates around a few tenths of a Kelvin The beam of droplets consistingof thousands of He atoms travels through a pickup cell where the atoms and molecules areloaded onto the droplet which will equilibrate again by vaporizing He atoms Under thesecircumstances metastable molecular states are formed as is demonstrated for alkali dimersALLARD et al from Graz report on new experiments on Rb2 and KRb where they observethe lowest triplet state of desorbing molecules from the droplet and additionally get the firstinformation about the distribution of populated molecular levels from the desorption Theaccompanying paper by BEUC et al from Zagreb gives valuable predictions for the expectedband profiles of absorption spectra from the metastable triplet state of KRb under the conditionsof such cold He droplets and will guide further experiments such as those from Graz The Hedroplets are a quite universal cold laboratory for a large manifold of molecules The groupin Rostock (see the paper by PRZYSTAWIK et al) applies it for silver dimers and reports on thebuilding of the metastable triplet state of Ag2 which is very interesting because Ag2 is locatedinside the He droplet whereas the alkali dimers sit on the surface of the droplet Thereforevery different interactions between the molecule and the droplet are expected This question isin view by the work of the group of F Stienkemeier at Freiburg (see the paper by CLAAS et al)where they studied K2 on the droplet and used the pumpndashprobe technique with femtosecondlaser pulses They find differences between the dynamics of photoionization in the gas phaseand on the droplet Also the timescale of the desorption process is identified

Supersonic expansion of gases other than He can lead to beams cooled in the translationalvelocity spread and internal energy to temperatures below 1 K Such cold samples are studiedfor weakly bound systems like van der Waals molecules KOPERSKI AND FRY apply this techniquefor spectroscopic studies of some metal noble gas compounds and especially to Hg2 for whichspectroscopic data are valuable for photoassociation experiments of trapped Hg atoms

Refrigerating is not universal slowing molecules

Parallel to the intensive studies of laser cooling of atoms like alkalis alkaline-earth elements anda few others or to the application of a coolant to widen the scope of molecules researchers haveaimed at creating slow molecules starting from fast molecular beams The main achievementsconcern the Stark deceleration technique [57] and the selection of slow molecules out of thefast ones [58] using guiding electric quadrupole fields With such techniques polar moleculeslike ND3 OH or CH2CO were successfully decelerated or selected Let us note that the heavypolar molecule YbF has been slowed down using the Stark deceleration approach for the questof permanent electric dipole moment of the electron [59]

New advances are reported by VAN DER MEERAKKER et al at the FHI in Berlin deceleratingNH in its metastable state a1 then converted inside a magnetic trap to the ground state X3minusby which accumulation of several decelerated packages could be achieved Additionally theypropose a nice idea for compressing the phase space by dumping to the ground state witha cw laser instead of a pulsed laser naturally assumed for packages arriving periodically intime With light fields it is not very difficult to obtain huge electric field strength by focussingThus in the paper by FULTON et al from Edinburgh deceleration is studied with a periodicoptical lattice from pulsed laser fields for NO and benzene reducing the kinetic energy by

Introductory Review

as much as 75 of the incoming one This might open research to much wider classes ofmolecules The paper by JUNG et al brings applications of the decelerated SO2 molecule intoview They measured the electric dipole moment of an excited state which they propose to usein a photodissociation experiment to produce cold radicals such as SO and O and the electricdipole moment could allow tuning of the photodissociation and thus the kinetic energy of thefragments

New recipes

After the fast development of different cooling recipes we need recipes and concepts forapplications With ultracold atomic ensembles in optical lattices one is able to model differentvariants of the BosendashHubbard system eg the transition from the Mott-insulator to thesuperfluid phase and the creation of ultracold dipolar molecular gases will add an importantnew ingredient by the anisotropy of the long range interaction to the richness of possible phasetransitions

But ultracold molecules opened the presently emerging field of ultracold chemistry witheither quantum effects (superchemistry with degenerate gases) and control of interactions withexternal fields (laser or static magnetic and electric fields) being new recipes in chemistry Firstexperimental observations of ultracold atomndashmolecule reactions have been reported [60 61]studying the lifetime of Cs2 ensembles embedded in Cs ensembles More theoretical modelsare required with which good experimental systems can be proposed or guided in future Thetheoretical work by WECK AND BALAKRISHNAN gives a general introduction to the physicalsituation of atomndashmolecule reactions at threshold and emphasizes the importance of theappearance of resonances otherwise low reaction rates are normally expected In the paper byYANG et al the link between ultracold collisions of atoms and molecules for quenching and theexperiments of buffer gas cooling mentioned above is presented for the example of rotationalquenching of CO through collision with H He or H2 in a wide range of energies from 10 μKto about 10 K as needed in the different stages of cooling processes The paper by GONZALEZ-SANCHEZ et al reports on similar aspects on the even simpler system OHminus in its ground state1+ with He atoms in 1S0 to be as complete as possible in the theoretical modelling Allthese calculations give rate constants which are of a magnitude to be observable in upcomingexperiments ultracold chemistry is probably richer than is thought by many chemists

Ion crystals but not traditional salt of the kitchen

Ion reactions play an important role in chemistry and for the cold regime the kitchen withinthe cold interstellar clouds comes immediately into view as a highly diluted system Thustrapped molecular ions certainly belong to a representation of the status of research of coldmolecules and their applications The group at Dusseldorf demonstrates in the paper by ROTH

et al the general applicability of sympathetic cooling of reaction partners like noble gas ionsor N+

2 and O+2 by laser cooled Be+ ions to produce a large variety of new cold ion samples eg

H+3 N2H+ etc These may serve for high-precision measurements for fundamental physical

questions like the time variation of fundamental constants or for studies of reaction processesrelevant in interstellar chemistry

Because of the strong Coulomb interaction between ions they appear regularly as Coulombcrystals if sufficiently cooled which gives great advantages because the ions within a crystalare well localized and thus addressable for particle by particle interaction and single particledetection To be quantum state specific in such studies the preparation of the initial physical

Introductory Review

condition is of fundamental importance This problem is addressed by two papers of theDrewsen group at Arhus In the first one by VOGELIUS et al they theoretically study the case ofrotational cooling of a polar molecule (here an ion which has an electric dipole moment withreference to its centre of mass) by optical coupling of the collective modes of a two-ion systemAs an example they describe the situation of MgH+ which is easily sympathetically cooled byCa+ In a second paper by the same authors they explore the possibility for probabilistic statepreparation in single molecular ions by projection measurements

Conclusion

Cold molecules offer a fantastic opportunity to develop strongly interdisciplinary research Twodomains are presently in view condensed matter physics with strongly correlated particlesand chemistry and superchemistry with the clear appearance of quantum effects in reactionprocesses This special issue on lsquoCold Moleculesrsquo gives insight into present lsquocookingrsquo inthe different laboratories which still concentrates on developing new methods and schemesto eventually proceed to systems which are most appropriate for the physical and chemicalquestions and applications When research on cold molecules started many scientists thought itwill be an artificial side-field of short duration But looking back the applicability of ultracoldatomic ensembles to model ideal solid state physics was not obvious Similarly the possiblerichness of superchemistry is unexplored but calculations of some systems clearly indicate thatthere is great hope of exciting results To harvest the fruits is probably more difficult becauseof the large number of degrees of freedom which we have to address by elegant experimentaland theoretical ideas and techniques not to be lost in the overwhelming routes which naturein principle has at its disposal and selects by evolution during millions of years We have torecognize the finite human lifetime Thus a thorough study of the papers provided by excellentlaboratories will be the right way of developing ideas during the short time available to us Wethank the authors for their contributions

Acknowledgments

The authors of this editorial acknowledge support from the Research and training Networkof the European Commission lsquoCold Moleculesrsquo (Contract HPRN- CT-2002-00290) and ODfrom the European Science Foundation through the network lsquoCollisions in Atom trapsrsquo

References

[1] Heinzen D J Wynar R Drummond P D and Kheruntsyan K V 2000 Superchemistry Dynamics of coupledatomic and molecular bose-einstein condensates Phys Rev Lett 84 5029

[2] httpwwwlacu-psudfrcoldmolecules[3] httpphysicsopenacukcats[4] httpquantendynamikphysikuni-freiburgdeles houches 2006[5] Chu S 1998 Nobel lecture the manipulation of neutral particles Rev Mod Phys 70 685[6] Cohen-Tannoudji C N 1998 Nobel lecture manipulating atoms with photons Rev Mod Phys 70 707[7] Phillips W D 1998 Nobel lecture laser cooling and trapping of neutral atoms Rev Mod Phys 70 721[8] Cornell E A and Wieman C E 2002 Nobel lecture BosendashEinstein condensation in a dilute gas the first 70 years

and some recent experiments Rev Mod Phys 74 875[9] Ketterle W 2002 Nobel lecture when atoms behaves as waves BosendashEinstein condensation and the atom laser

Rev Mod Phys 74 1131[10] Weiner J Bagnato V S Zilio S C and Julienne P S 1999 Experiments and theory in cold and ultracold collisions

Rev Mod Phys 71 1

Introductory Review

[11] Schloder U Engler H Schunemann U Grimmand R and Weidemuller M 1999 Cold inelastic collisions betweenlithium and cesium in a two- species magneto-optical trap Eur Phys J D 7 331

[12] Shaffer J P Chalupczak W and Bigelow N P 1999 Highly-excited states of ultracold molecules photoassociativespectroscopy of Na2 Phys Rev Lett 83 3621

[13] Mancini M W Telles G D Caires A R L Bagnato V S and Marcassa L G 2004 Observation of ultracoldground-state heteronuclear molecules Phys Rev Lett 92 133203

[14] Thorsheim H R Weiner J and Julienne P S 1987 Laser-induced photoassociation of ultracold sodium atomsPhys Rev Lett 58 2420

[15] Lett P D Helmerson K Philips W D Ratliff L P RolstonS L and Wagshul M E 1993 Spectroscopy of Na2 byphotoassociation of laser-cooled Na Phys Rev Lett 71 2200

[16] Miller J D Cline R A and Heinzen D J 1993 Photoassociation spectrum of ultracold Rb atoms Phys Rev Lett71 2204

[17] Azizi S Aymar M and Dulieu O 2004 Prospects for the formation of ultracold ground state polar moleculesfrom mixed alkali atom pair Eur Phys J D 31 195

[18] Lett P D Julienne P S and Philips W D 1995 Photoassociative spectroscopy of laser-cooled atoms Annu RevPhys Chem 46 423

[19] Stwalley W C and Wang H 1999 Photoassociation of ultracold atoms a new spectroscopic technique J MolSpectrosc 195 194

[20] Jones K M Tiesinga E Lett P D and Julienne P S 2006 Ultracold photoassociation spectroscopy Long-rangemolecules and atomic scattering Rep Prog Phys 78 483

[21] Stwalley W C Uang Y H and Pichler G 1978 Pure long-range molecules Phys Rev Lett 41 1164[22] Bohn J L and Julienne P S 1996 Semianalytic treatment of two-color photoassociation spectroscopy and control

of cold atoms Phys Rev A 54 R4637[23] Bohn J L and Julienne P S 1999 Semianalytic theory of laser-assisted resonant cold collisions Phys Rev A 60

414[24] Fioretti A Comparat D Crubellier A Dulieu O Masnou-Seeuws F and Pillet P 1998 Formation of Cs2 cold

molecules through photoassociation Phys Rev Lett 80 4402[25] C Gabbanini A Fioretti A Lucchesini S Gozzini and M Mazzoni 2000 Cold rubidium molecules formed in a

magneto-optical trap Phys Rev Lett 84 2814[26] Dion C M Drag C Dulieu O Laburthe Tolra B Masnou-Seeuws F and Pillet P 2001 Resonant coupling in the

formation of ultracold ground state molecules via photoassociation Phys Rev Lett 86 2253[27] Fatemi F K Jones K M Lett P D and Tiesinga E 2002 Ultracold ground-state molecule production in sodium

Phys Rev A 66 053401[28] Nikolov A N Eyler E E Wang X T Li J Wang H Stwalley W C and Gould P L 1999 Observation of ultracold

ground state potassium molecules Phys Rev Lett 82 703[29] Nikolov A N Enscher J R Eyler E E Wang H Stwalley W C and Gould P L 2000 Efficient production of ground

state potassium molecules at sub-mK temperatures by two-step photoassociation Phys Rev Lett 84 246[30] Band Y B and Julienne P S 1995 Ultracold molecule production by laser-cooled atom photoassociation Phys

Rev A 51 R4317[31] Lozeille J Fioretti A Gabbanini C Huang Y Pechkis H K Wang D Gould P L Eyler E E Stwalley W C

Aymar M and Dulieu O 2006 Detection of cold Rb2 triplet state molecules by two-photon ionization EurPhys J D 39

[32] Damski B Santos L Tiemann E Lewenstein M Kotochigova S Julienne P and Zoller P 2003 Creation of adipolar superfluid in optical lattices Phys Rev Lett 90 110401

[33] DeMille D 2002 Quantum computation with trapped polar molecules Phys Rev Lett 88 067901[34] Kallush S Segev B and Cote R 2005 Evanescent-wave mirror for ultracold diatomic polar molecules Phys Rev

Lett 95 163005[35] Sandars P G H 1967 Measurability of th eproton electric dipole moment Phys Rev Lett 19 1396[36] Hudson J J Sauer B E Tarbutt M R and Hinds E A 2002 Measurement of the electron electric dipole moment

using YbF molecules Phys Rev Lett 89 023003[37] Kawall D Bay F Bickman S Jiang Y and DeMille D 2004 Precision ZeemanndashStark spectroscopy of the

metastable a(1)3σ + state of PbO Phys Rev Lett 92 133007[38] Doyle J Friedrich B Krems R V and Masnou-Seeuws F 2004 Special issue on ultracold polar molecules 2004

Eur Phys J D 31 149 and following papers[39] Wang H Gould P L and Stwalley W C 1998 Fine-structure predissociation of ultracold photoassociated39K2

molecules observed by fragmentation spectroscopy Phys Rev Lett 80 476[40] Kerman A J Sage J M Sainis S Bergeman T and DeMille D 2004 Production and state-selective detection of

ultracold rbcs molecules Phys Rev Lett 92 153001

Introductory Review

[41] Wang D Qi J Stone M F Nikolayeva O Wang H Hattaway B Gensemer S D Gould P L Eyler E E andStwalley W C 2004 Photoassociative production and trapping of ultracold krb molecules Phys Rev Lett 93243005

[42] Sage J M Sainis S Bergeman T and DeMille D 2005 Optical production of ultracold polar molecules PhysRev Lett 94 203001

[43] Wang D Qi J Stone M F Nikolayeva O Hattaway B Gensemer S D Wang H Zemke W T Gould P LEyler E E and Stwalley W C 2004 The photoassociative spectroscopy photoassociative molecule formationand trapping of ultracold 39K85Rb Eur Phys J D 31 165

[44] Orozco L 2006 private communication[45] Sprouse G D Fliller R P Grossman J S Orozco L A and Pearson M R 2002 Traps for neutral radioactive atoms

Nucl Phys A 701 597[46] Abraham E R I McAlexander W I Sackett C A and Hulet R G 1995 Spectroscopic determination of the s-wave

scattering length of lithium Phys Rev Lett 74 1315[47] Tsai C C Freeland R S Vogels J M Boesten H M J M Gardner J R Heinzen D J and Verhaar B J 1997

Two-color photoassociation spectroscopy of ground state Rb2 Phys Rev Lett 79 1245[48] Laburthe Tolra B Drag C and Pillet P 2001 Formation of cold cesium molecules through stimulated raman

photoassociation Phys Rev A 64 R61401[49] Machholm M Giusti-Suzor A and Mies F H 1994 Photoassociation of atoms in ultracold collisions probed

bywave-packet dynamics Phys Rev A 50 5025[50] Judson R and Rabitz H 1992 Teaching lasers to control molecules Phys Rev Lett 68 1500[51] Vala J Dulieu O Masnou-Seeuws F and Kosloff R 2001 Coherent control of cold molecule formation through

photoassociation using a chirped pulsed laser field Phys Rev A 63 013412[52] Koch C P Luc-Koenig E and Masnou-Seeuws F 2006 Making ultracold molecules in a two color pump-dump

photoassociation scheme using chirped pulses Phys Rev A 73 033408[53] Wright M J Gensemer S D Vala J Kosloff R and Gould P L 2005 Control of ultracold collisions with frequency-

chirped light Phys Rev Lett 95 063001[54] Brown B L Dicks A J and Walmsley I A 2006 Coherent control of ultracold molecule dynamics in a magneto-

optical trap by use of chirped femtosecond laser pulses Phys Rev Lett 96 173002[55] Salzmann W Poschinger U Wester R Weidemuller M Merli A Weber S M Sauer F Plewicki M Weise F

Esparza A M Woste L and Lindinger A 2006 Coherent control with shaped femtosecond laser pulses appliedto ultracold molecules Phys Rev A 73 023414

[56] Weinstein J D deCarvalho R Kim J Patterson D Friedrich B and Doyle J M 1998 Magnetic trapping of atomicchromium Phys Rev A 57 R3173

[57] Bethlem H L and Meijer G 2003 Production and application of translationally cold molecules Int Rev PhysChem 22 73

[58] Rangwala S A Junglen T Rieger T Pinkse P W H and Rempe G 2003 Contnuous source of translationally colddipolar molecules Phys Rev A 64 043406

[59] Tarbutt M R Bethlem H L Hudson J J Ryabov V L Ryzhov V A Sauer B E Meijer G and Hinds E A 2004Slowing heavy ground-state molecules using an alternating gradient decelerator Phys Rev Lett 92 173002

[60] Staanum P Kraft S D Lange J Wester R and Weidemuller M 2006 Experimental investigation of ultracoldatom-molecule collisions Phys Rev Lett 96 023201

[61] Zahzam N Vogt T Mudrich M Comparat D and Pillet P 2006 Atomndashmolecule collisions in an optically trappedgas Phys Rev Lett 96 023202

Introductory Review

Time allows fine cooking processes under optimal control

The photoassociation route described above relies on cw lasers for the excitation step and onspontaneous emission for the stabilization step While producing large amounts of ultracoldmolecules the main drawback of this sequence is that ultracold molecules are generally createdin a broad distribution of vibrational levels most often well above the v = 0 level Inthis respect the situation described by VATASESCU et al is probably an exceptional case Analternative has been explored through a two-colour stimulated Raman scheme [46 47 48] butthe efficiency of the process is severely limited by the reversibility of the process ie theultracold molecules can be dissociated through the same scheme

The idea of using laser pulses to overcome this limitation rose some time ago in thecontext of cold atom photoassociation with the model proposed by Machholm et al [49]based on a pumpndashdump scheme It is easy to realize that the characteristics of the pulses egintensity duration time delay offer control parameters to optimize the process efficiency verymuch along the ideas of adaptive optimal control initiated by Judson and Rabitz [50] Morerecently chirped laser pulses have been considered theoretically for optimizing the formationof ultracold molecules in a pumpndashdump scheme [51 52] None of these possibilities have beenexperimentally demonstrated yet even if a certain degree of control of cold collisions [53] andof dissociation of ultracold Rb2 molecules with chirped laser pulses [54] has been observed aswell as of ultracold Rb2 dissociation with feedback control [55]

Four theoretical papers in this issue intend to model possible experiments with Rb2 dimersas this is the species chosen by the experimental groups above The paper by KOCH et alassesses the feasibility of a formation scheme with picoseconds chirped lasers by calculatingthe number of created molecules per pulse with three different models for taking in account theinitial velocity distribution of the atoms POSCHINGER et al and BROWN AND WALMSLEY explorecomplementary ways to design shaped femtosecond pulses the former uses adaptive feedbackcontrol of the pulse amplitude and phase with a genetic algorithm while the latter focuses onintuitive pulse designs which could provide a starting point for a future experiment FinallySCHAFER-BUNG et al assume that the PA step is achieved via cw lasers creating an excited Rb2

molecule in a well-defined vibrational level and concentrate on the stabilization step througha pulse sequence designed by an optimal control procedure

In the absence of experiments approaching the proposed models above it is too early tocomment on the quality of these models However the theoretical investigations undoubtlyinclude more and more refinements which will certainly be very useful for guiding upcomingexperiments

lsquoRefrigeratingrsquo molecules

The laser schemes are very efficient in cooling but are not generally applicable at least formolecules For atoms the limits mainly come from the possible unavailability of laser sourcesfor exciting a good atomic transition But for molecules the wide spread of the spontaneousdecay into different quantum states limits the process to a few cooling cycles before themolecule is lost in a state which is not in resonance with the applied field even if one overlapsseveral light fields Chirped lasers are still to be explored for this application

As in solid state physics several cooling steps could progress to the desired goal ofultracold ensembles As a first step buffer-gas cooling is an idea which was successfullyapplied for the molecule CaH by the Doyle group at Harvard [56] which because of itsparamagnetic ground state could be magnetically trapped at temperatures around 100 mKOther molecules could be cooled with this method but not trapped so far In their paper

Introductory Review

BAKKER et al from the Berlin group of A Peters report a new setup for buffer-gas cooling andcould successfully demonstrate the cooling and trapping of Cr atoms The paper discusses indetail the necessary conditions for trapping paramagnetic molecules after buffer-gas coolingand proposes as good candidates CrH and MnH with their high spin ground states and not toolarge spinndashspin interactions

A very different cooling device for molecules is the helium nanodroplet expanding out ofa nozzle which equilibrates around a few tenths of a Kelvin The beam of droplets consistingof thousands of He atoms travels through a pickup cell where the atoms and molecules areloaded onto the droplet which will equilibrate again by vaporizing He atoms Under thesecircumstances metastable molecular states are formed as is demonstrated for alkali dimersALLARD et al from Graz report on new experiments on Rb2 and KRb where they observethe lowest triplet state of desorbing molecules from the droplet and additionally get the firstinformation about the distribution of populated molecular levels from the desorption Theaccompanying paper by BEUC et al from Zagreb gives valuable predictions for the expectedband profiles of absorption spectra from the metastable triplet state of KRb under the conditionsof such cold He droplets and will guide further experiments such as those from Graz The Hedroplets are a quite universal cold laboratory for a large manifold of molecules The groupin Rostock (see the paper by PRZYSTAWIK et al) applies it for silver dimers and reports on thebuilding of the metastable triplet state of Ag2 which is very interesting because Ag2 is locatedinside the He droplet whereas the alkali dimers sit on the surface of the droplet Thereforevery different interactions between the molecule and the droplet are expected This question isin view by the work of the group of F Stienkemeier at Freiburg (see the paper by CLAAS et al)where they studied K2 on the droplet and used the pumpndashprobe technique with femtosecondlaser pulses They find differences between the dynamics of photoionization in the gas phaseand on the droplet Also the timescale of the desorption process is identified

Supersonic expansion of gases other than He can lead to beams cooled in the translationalvelocity spread and internal energy to temperatures below 1 K Such cold samples are studiedfor weakly bound systems like van der Waals molecules KOPERSKI AND FRY apply this techniquefor spectroscopic studies of some metal noble gas compounds and especially to Hg2 for whichspectroscopic data are valuable for photoassociation experiments of trapped Hg atoms

Refrigerating is not universal slowing molecules

Parallel to the intensive studies of laser cooling of atoms like alkalis alkaline-earth elements anda few others or to the application of a coolant to widen the scope of molecules researchers haveaimed at creating slow molecules starting from fast molecular beams The main achievementsconcern the Stark deceleration technique [57] and the selection of slow molecules out of thefast ones [58] using guiding electric quadrupole fields With such techniques polar moleculeslike ND3 OH or CH2CO were successfully decelerated or selected Let us note that the heavypolar molecule YbF has been slowed down using the Stark deceleration approach for the questof permanent electric dipole moment of the electron [59]

New advances are reported by VAN DER MEERAKKER et al at the FHI in Berlin deceleratingNH in its metastable state a1 then converted inside a magnetic trap to the ground state X3minusby which accumulation of several decelerated packages could be achieved Additionally theypropose a nice idea for compressing the phase space by dumping to the ground state witha cw laser instead of a pulsed laser naturally assumed for packages arriving periodically intime With light fields it is not very difficult to obtain huge electric field strength by focussingThus in the paper by FULTON et al from Edinburgh deceleration is studied with a periodicoptical lattice from pulsed laser fields for NO and benzene reducing the kinetic energy by

Introductory Review

as much as 75 of the incoming one This might open research to much wider classes ofmolecules The paper by JUNG et al brings applications of the decelerated SO2 molecule intoview They measured the electric dipole moment of an excited state which they propose to usein a photodissociation experiment to produce cold radicals such as SO and O and the electricdipole moment could allow tuning of the photodissociation and thus the kinetic energy of thefragments

New recipes

After the fast development of different cooling recipes we need recipes and concepts forapplications With ultracold atomic ensembles in optical lattices one is able to model differentvariants of the BosendashHubbard system eg the transition from the Mott-insulator to thesuperfluid phase and the creation of ultracold dipolar molecular gases will add an importantnew ingredient by the anisotropy of the long range interaction to the richness of possible phasetransitions

But ultracold molecules opened the presently emerging field of ultracold chemistry witheither quantum effects (superchemistry with degenerate gases) and control of interactions withexternal fields (laser or static magnetic and electric fields) being new recipes in chemistry Firstexperimental observations of ultracold atomndashmolecule reactions have been reported [60 61]studying the lifetime of Cs2 ensembles embedded in Cs ensembles More theoretical modelsare required with which good experimental systems can be proposed or guided in future Thetheoretical work by WECK AND BALAKRISHNAN gives a general introduction to the physicalsituation of atomndashmolecule reactions at threshold and emphasizes the importance of theappearance of resonances otherwise low reaction rates are normally expected In the paper byYANG et al the link between ultracold collisions of atoms and molecules for quenching and theexperiments of buffer gas cooling mentioned above is presented for the example of rotationalquenching of CO through collision with H He or H2 in a wide range of energies from 10 μKto about 10 K as needed in the different stages of cooling processes The paper by GONZALEZ-SANCHEZ et al reports on similar aspects on the even simpler system OHminus in its ground state1+ with He atoms in 1S0 to be as complete as possible in the theoretical modelling Allthese calculations give rate constants which are of a magnitude to be observable in upcomingexperiments ultracold chemistry is probably richer than is thought by many chemists

Ion crystals but not traditional salt of the kitchen

Ion reactions play an important role in chemistry and for the cold regime the kitchen withinthe cold interstellar clouds comes immediately into view as a highly diluted system Thustrapped molecular ions certainly belong to a representation of the status of research of coldmolecules and their applications The group at Dusseldorf demonstrates in the paper by ROTH

et al the general applicability of sympathetic cooling of reaction partners like noble gas ionsor N+

2 and O+2 by laser cooled Be+ ions to produce a large variety of new cold ion samples eg

H+3 N2H+ etc These may serve for high-precision measurements for fundamental physical

questions like the time variation of fundamental constants or for studies of reaction processesrelevant in interstellar chemistry

Because of the strong Coulomb interaction between ions they appear regularly as Coulombcrystals if sufficiently cooled which gives great advantages because the ions within a crystalare well localized and thus addressable for particle by particle interaction and single particledetection To be quantum state specific in such studies the preparation of the initial physical

Introductory Review

condition is of fundamental importance This problem is addressed by two papers of theDrewsen group at Arhus In the first one by VOGELIUS et al they theoretically study the case ofrotational cooling of a polar molecule (here an ion which has an electric dipole moment withreference to its centre of mass) by optical coupling of the collective modes of a two-ion systemAs an example they describe the situation of MgH+ which is easily sympathetically cooled byCa+ In a second paper by the same authors they explore the possibility for probabilistic statepreparation in single molecular ions by projection measurements

Conclusion

Cold molecules offer a fantastic opportunity to develop strongly interdisciplinary research Twodomains are presently in view condensed matter physics with strongly correlated particlesand chemistry and superchemistry with the clear appearance of quantum effects in reactionprocesses This special issue on lsquoCold Moleculesrsquo gives insight into present lsquocookingrsquo inthe different laboratories which still concentrates on developing new methods and schemesto eventually proceed to systems which are most appropriate for the physical and chemicalquestions and applications When research on cold molecules started many scientists thought itwill be an artificial side-field of short duration But looking back the applicability of ultracoldatomic ensembles to model ideal solid state physics was not obvious Similarly the possiblerichness of superchemistry is unexplored but calculations of some systems clearly indicate thatthere is great hope of exciting results To harvest the fruits is probably more difficult becauseof the large number of degrees of freedom which we have to address by elegant experimentaland theoretical ideas and techniques not to be lost in the overwhelming routes which naturein principle has at its disposal and selects by evolution during millions of years We have torecognize the finite human lifetime Thus a thorough study of the papers provided by excellentlaboratories will be the right way of developing ideas during the short time available to us Wethank the authors for their contributions

Acknowledgments

The authors of this editorial acknowledge support from the Research and training Networkof the European Commission lsquoCold Moleculesrsquo (Contract HPRN- CT-2002-00290) and ODfrom the European Science Foundation through the network lsquoCollisions in Atom trapsrsquo

References

[1] Heinzen D J Wynar R Drummond P D and Kheruntsyan K V 2000 Superchemistry Dynamics of coupledatomic and molecular bose-einstein condensates Phys Rev Lett 84 5029

[2] httpwwwlacu-psudfrcoldmolecules[3] httpphysicsopenacukcats[4] httpquantendynamikphysikuni-freiburgdeles houches 2006[5] Chu S 1998 Nobel lecture the manipulation of neutral particles Rev Mod Phys 70 685[6] Cohen-Tannoudji C N 1998 Nobel lecture manipulating atoms with photons Rev Mod Phys 70 707[7] Phillips W D 1998 Nobel lecture laser cooling and trapping of neutral atoms Rev Mod Phys 70 721[8] Cornell E A and Wieman C E 2002 Nobel lecture BosendashEinstein condensation in a dilute gas the first 70 years

and some recent experiments Rev Mod Phys 74 875[9] Ketterle W 2002 Nobel lecture when atoms behaves as waves BosendashEinstein condensation and the atom laser

Rev Mod Phys 74 1131[10] Weiner J Bagnato V S Zilio S C and Julienne P S 1999 Experiments and theory in cold and ultracold collisions

Rev Mod Phys 71 1

Introductory Review

[11] Schloder U Engler H Schunemann U Grimmand R and Weidemuller M 1999 Cold inelastic collisions betweenlithium and cesium in a two- species magneto-optical trap Eur Phys J D 7 331

[12] Shaffer J P Chalupczak W and Bigelow N P 1999 Highly-excited states of ultracold molecules photoassociativespectroscopy of Na2 Phys Rev Lett 83 3621

[13] Mancini M W Telles G D Caires A R L Bagnato V S and Marcassa L G 2004 Observation of ultracoldground-state heteronuclear molecules Phys Rev Lett 92 133203

[14] Thorsheim H R Weiner J and Julienne P S 1987 Laser-induced photoassociation of ultracold sodium atomsPhys Rev Lett 58 2420

[15] Lett P D Helmerson K Philips W D Ratliff L P RolstonS L and Wagshul M E 1993 Spectroscopy of Na2 byphotoassociation of laser-cooled Na Phys Rev Lett 71 2200

[16] Miller J D Cline R A and Heinzen D J 1993 Photoassociation spectrum of ultracold Rb atoms Phys Rev Lett71 2204

[17] Azizi S Aymar M and Dulieu O 2004 Prospects for the formation of ultracold ground state polar moleculesfrom mixed alkali atom pair Eur Phys J D 31 195

[18] Lett P D Julienne P S and Philips W D 1995 Photoassociative spectroscopy of laser-cooled atoms Annu RevPhys Chem 46 423

[19] Stwalley W C and Wang H 1999 Photoassociation of ultracold atoms a new spectroscopic technique J MolSpectrosc 195 194

[20] Jones K M Tiesinga E Lett P D and Julienne P S 2006 Ultracold photoassociation spectroscopy Long-rangemolecules and atomic scattering Rep Prog Phys 78 483

[21] Stwalley W C Uang Y H and Pichler G 1978 Pure long-range molecules Phys Rev Lett 41 1164[22] Bohn J L and Julienne P S 1996 Semianalytic treatment of two-color photoassociation spectroscopy and control

of cold atoms Phys Rev A 54 R4637[23] Bohn J L and Julienne P S 1999 Semianalytic theory of laser-assisted resonant cold collisions Phys Rev A 60

414[24] Fioretti A Comparat D Crubellier A Dulieu O Masnou-Seeuws F and Pillet P 1998 Formation of Cs2 cold

molecules through photoassociation Phys Rev Lett 80 4402[25] C Gabbanini A Fioretti A Lucchesini S Gozzini and M Mazzoni 2000 Cold rubidium molecules formed in a

magneto-optical trap Phys Rev Lett 84 2814[26] Dion C M Drag C Dulieu O Laburthe Tolra B Masnou-Seeuws F and Pillet P 2001 Resonant coupling in the

formation of ultracold ground state molecules via photoassociation Phys Rev Lett 86 2253[27] Fatemi F K Jones K M Lett P D and Tiesinga E 2002 Ultracold ground-state molecule production in sodium

Phys Rev A 66 053401[28] Nikolov A N Eyler E E Wang X T Li J Wang H Stwalley W C and Gould P L 1999 Observation of ultracold

ground state potassium molecules Phys Rev Lett 82 703[29] Nikolov A N Enscher J R Eyler E E Wang H Stwalley W C and Gould P L 2000 Efficient production of ground

state potassium molecules at sub-mK temperatures by two-step photoassociation Phys Rev Lett 84 246[30] Band Y B and Julienne P S 1995 Ultracold molecule production by laser-cooled atom photoassociation Phys

Rev A 51 R4317[31] Lozeille J Fioretti A Gabbanini C Huang Y Pechkis H K Wang D Gould P L Eyler E E Stwalley W C

Aymar M and Dulieu O 2006 Detection of cold Rb2 triplet state molecules by two-photon ionization EurPhys J D 39

[32] Damski B Santos L Tiemann E Lewenstein M Kotochigova S Julienne P and Zoller P 2003 Creation of adipolar superfluid in optical lattices Phys Rev Lett 90 110401

[33] DeMille D 2002 Quantum computation with trapped polar molecules Phys Rev Lett 88 067901[34] Kallush S Segev B and Cote R 2005 Evanescent-wave mirror for ultracold diatomic polar molecules Phys Rev

Lett 95 163005[35] Sandars P G H 1967 Measurability of th eproton electric dipole moment Phys Rev Lett 19 1396[36] Hudson J J Sauer B E Tarbutt M R and Hinds E A 2002 Measurement of the electron electric dipole moment

using YbF molecules Phys Rev Lett 89 023003[37] Kawall D Bay F Bickman S Jiang Y and DeMille D 2004 Precision ZeemanndashStark spectroscopy of the

metastable a(1)3σ + state of PbO Phys Rev Lett 92 133007[38] Doyle J Friedrich B Krems R V and Masnou-Seeuws F 2004 Special issue on ultracold polar molecules 2004

Eur Phys J D 31 149 and following papers[39] Wang H Gould P L and Stwalley W C 1998 Fine-structure predissociation of ultracold photoassociated39K2

molecules observed by fragmentation spectroscopy Phys Rev Lett 80 476[40] Kerman A J Sage J M Sainis S Bergeman T and DeMille D 2004 Production and state-selective detection of

ultracold rbcs molecules Phys Rev Lett 92 153001

Introductory Review

[41] Wang D Qi J Stone M F Nikolayeva O Wang H Hattaway B Gensemer S D Gould P L Eyler E E andStwalley W C 2004 Photoassociative production and trapping of ultracold krb molecules Phys Rev Lett 93243005

[42] Sage J M Sainis S Bergeman T and DeMille D 2005 Optical production of ultracold polar molecules PhysRev Lett 94 203001

[43] Wang D Qi J Stone M F Nikolayeva O Hattaway B Gensemer S D Wang H Zemke W T Gould P LEyler E E and Stwalley W C 2004 The photoassociative spectroscopy photoassociative molecule formationand trapping of ultracold 39K85Rb Eur Phys J D 31 165

[44] Orozco L 2006 private communication[45] Sprouse G D Fliller R P Grossman J S Orozco L A and Pearson M R 2002 Traps for neutral radioactive atoms

Nucl Phys A 701 597[46] Abraham E R I McAlexander W I Sackett C A and Hulet R G 1995 Spectroscopic determination of the s-wave

scattering length of lithium Phys Rev Lett 74 1315[47] Tsai C C Freeland R S Vogels J M Boesten H M J M Gardner J R Heinzen D J and Verhaar B J 1997

Two-color photoassociation spectroscopy of ground state Rb2 Phys Rev Lett 79 1245[48] Laburthe Tolra B Drag C and Pillet P 2001 Formation of cold cesium molecules through stimulated raman

photoassociation Phys Rev A 64 R61401[49] Machholm M Giusti-Suzor A and Mies F H 1994 Photoassociation of atoms in ultracold collisions probed

bywave-packet dynamics Phys Rev A 50 5025[50] Judson R and Rabitz H 1992 Teaching lasers to control molecules Phys Rev Lett 68 1500[51] Vala J Dulieu O Masnou-Seeuws F and Kosloff R 2001 Coherent control of cold molecule formation through

photoassociation using a chirped pulsed laser field Phys Rev A 63 013412[52] Koch C P Luc-Koenig E and Masnou-Seeuws F 2006 Making ultracold molecules in a two color pump-dump

photoassociation scheme using chirped pulses Phys Rev A 73 033408[53] Wright M J Gensemer S D Vala J Kosloff R and Gould P L 2005 Control of ultracold collisions with frequency-

chirped light Phys Rev Lett 95 063001[54] Brown B L Dicks A J and Walmsley I A 2006 Coherent control of ultracold molecule dynamics in a magneto-

optical trap by use of chirped femtosecond laser pulses Phys Rev Lett 96 173002[55] Salzmann W Poschinger U Wester R Weidemuller M Merli A Weber S M Sauer F Plewicki M Weise F

Esparza A M Woste L and Lindinger A 2006 Coherent control with shaped femtosecond laser pulses appliedto ultracold molecules Phys Rev A 73 023414

[56] Weinstein J D deCarvalho R Kim J Patterson D Friedrich B and Doyle J M 1998 Magnetic trapping of atomicchromium Phys Rev A 57 R3173

[57] Bethlem H L and Meijer G 2003 Production and application of translationally cold molecules Int Rev PhysChem 22 73

[58] Rangwala S A Junglen T Rieger T Pinkse P W H and Rempe G 2003 Contnuous source of translationally colddipolar molecules Phys Rev A 64 043406

[59] Tarbutt M R Bethlem H L Hudson J J Ryabov V L Ryzhov V A Sauer B E Meijer G and Hinds E A 2004Slowing heavy ground-state molecules using an alternating gradient decelerator Phys Rev Lett 92 173002

[60] Staanum P Kraft S D Lange J Wester R and Weidemuller M 2006 Experimental investigation of ultracoldatom-molecule collisions Phys Rev Lett 96 023201

[61] Zahzam N Vogt T Mudrich M Comparat D and Pillet P 2006 Atomndashmolecule collisions in an optically trappedgas Phys Rev Lett 96 023202

Introductory Review

BAKKER et al from the Berlin group of A Peters report a new setup for buffer-gas cooling andcould successfully demonstrate the cooling and trapping of Cr atoms The paper discusses indetail the necessary conditions for trapping paramagnetic molecules after buffer-gas coolingand proposes as good candidates CrH and MnH with their high spin ground states and not toolarge spinndashspin interactions

A very different cooling device for molecules is the helium nanodroplet expanding out ofa nozzle which equilibrates around a few tenths of a Kelvin The beam of droplets consistingof thousands of He atoms travels through a pickup cell where the atoms and molecules areloaded onto the droplet which will equilibrate again by vaporizing He atoms Under thesecircumstances metastable molecular states are formed as is demonstrated for alkali dimersALLARD et al from Graz report on new experiments on Rb2 and KRb where they observethe lowest triplet state of desorbing molecules from the droplet and additionally get the firstinformation about the distribution of populated molecular levels from the desorption Theaccompanying paper by BEUC et al from Zagreb gives valuable predictions for the expectedband profiles of absorption spectra from the metastable triplet state of KRb under the conditionsof such cold He droplets and will guide further experiments such as those from Graz The Hedroplets are a quite universal cold laboratory for a large manifold of molecules The groupin Rostock (see the paper by PRZYSTAWIK et al) applies it for silver dimers and reports on thebuilding of the metastable triplet state of Ag2 which is very interesting because Ag2 is locatedinside the He droplet whereas the alkali dimers sit on the surface of the droplet Thereforevery different interactions between the molecule and the droplet are expected This question isin view by the work of the group of F Stienkemeier at Freiburg (see the paper by CLAAS et al)where they studied K2 on the droplet and used the pumpndashprobe technique with femtosecondlaser pulses They find differences between the dynamics of photoionization in the gas phaseand on the droplet Also the timescale of the desorption process is identified

Supersonic expansion of gases other than He can lead to beams cooled in the translationalvelocity spread and internal energy to temperatures below 1 K Such cold samples are studiedfor weakly bound systems like van der Waals molecules KOPERSKI AND FRY apply this techniquefor spectroscopic studies of some metal noble gas compounds and especially to Hg2 for whichspectroscopic data are valuable for photoassociation experiments of trapped Hg atoms

Refrigerating is not universal slowing molecules

Parallel to the intensive studies of laser cooling of atoms like alkalis alkaline-earth elements anda few others or to the application of a coolant to widen the scope of molecules researchers haveaimed at creating slow molecules starting from fast molecular beams The main achievementsconcern the Stark deceleration technique [57] and the selection of slow molecules out of thefast ones [58] using guiding electric quadrupole fields With such techniques polar moleculeslike ND3 OH or CH2CO were successfully decelerated or selected Let us note that the heavypolar molecule YbF has been slowed down using the Stark deceleration approach for the questof permanent electric dipole moment of the electron [59]

New advances are reported by VAN DER MEERAKKER et al at the FHI in Berlin deceleratingNH in its metastable state a1 then converted inside a magnetic trap to the ground state X3minusby which accumulation of several decelerated packages could be achieved Additionally theypropose a nice idea for compressing the phase space by dumping to the ground state witha cw laser instead of a pulsed laser naturally assumed for packages arriving periodically intime With light fields it is not very difficult to obtain huge electric field strength by focussingThus in the paper by FULTON et al from Edinburgh deceleration is studied with a periodicoptical lattice from pulsed laser fields for NO and benzene reducing the kinetic energy by

Introductory Review

as much as 75 of the incoming one This might open research to much wider classes ofmolecules The paper by JUNG et al brings applications of the decelerated SO2 molecule intoview They measured the electric dipole moment of an excited state which they propose to usein a photodissociation experiment to produce cold radicals such as SO and O and the electricdipole moment could allow tuning of the photodissociation and thus the kinetic energy of thefragments

New recipes

After the fast development of different cooling recipes we need recipes and concepts forapplications With ultracold atomic ensembles in optical lattices one is able to model differentvariants of the BosendashHubbard system eg the transition from the Mott-insulator to thesuperfluid phase and the creation of ultracold dipolar molecular gases will add an importantnew ingredient by the anisotropy of the long range interaction to the richness of possible phasetransitions

But ultracold molecules opened the presently emerging field of ultracold chemistry witheither quantum effects (superchemistry with degenerate gases) and control of interactions withexternal fields (laser or static magnetic and electric fields) being new recipes in chemistry Firstexperimental observations of ultracold atomndashmolecule reactions have been reported [60 61]studying the lifetime of Cs2 ensembles embedded in Cs ensembles More theoretical modelsare required with which good experimental systems can be proposed or guided in future Thetheoretical work by WECK AND BALAKRISHNAN gives a general introduction to the physicalsituation of atomndashmolecule reactions at threshold and emphasizes the importance of theappearance of resonances otherwise low reaction rates are normally expected In the paper byYANG et al the link between ultracold collisions of atoms and molecules for quenching and theexperiments of buffer gas cooling mentioned above is presented for the example of rotationalquenching of CO through collision with H He or H2 in a wide range of energies from 10 μKto about 10 K as needed in the different stages of cooling processes The paper by GONZALEZ-SANCHEZ et al reports on similar aspects on the even simpler system OHminus in its ground state1+ with He atoms in 1S0 to be as complete as possible in the theoretical modelling Allthese calculations give rate constants which are of a magnitude to be observable in upcomingexperiments ultracold chemistry is probably richer than is thought by many chemists

Ion crystals but not traditional salt of the kitchen

Ion reactions play an important role in chemistry and for the cold regime the kitchen withinthe cold interstellar clouds comes immediately into view as a highly diluted system Thustrapped molecular ions certainly belong to a representation of the status of research of coldmolecules and their applications The group at Dusseldorf demonstrates in the paper by ROTH

et al the general applicability of sympathetic cooling of reaction partners like noble gas ionsor N+

2 and O+2 by laser cooled Be+ ions to produce a large variety of new cold ion samples eg

H+3 N2H+ etc These may serve for high-precision measurements for fundamental physical

questions like the time variation of fundamental constants or for studies of reaction processesrelevant in interstellar chemistry

Because of the strong Coulomb interaction between ions they appear regularly as Coulombcrystals if sufficiently cooled which gives great advantages because the ions within a crystalare well localized and thus addressable for particle by particle interaction and single particledetection To be quantum state specific in such studies the preparation of the initial physical

Introductory Review

condition is of fundamental importance This problem is addressed by two papers of theDrewsen group at Arhus In the first one by VOGELIUS et al they theoretically study the case ofrotational cooling of a polar molecule (here an ion which has an electric dipole moment withreference to its centre of mass) by optical coupling of the collective modes of a two-ion systemAs an example they describe the situation of MgH+ which is easily sympathetically cooled byCa+ In a second paper by the same authors they explore the possibility for probabilistic statepreparation in single molecular ions by projection measurements

Conclusion

Cold molecules offer a fantastic opportunity to develop strongly interdisciplinary research Twodomains are presently in view condensed matter physics with strongly correlated particlesand chemistry and superchemistry with the clear appearance of quantum effects in reactionprocesses This special issue on lsquoCold Moleculesrsquo gives insight into present lsquocookingrsquo inthe different laboratories which still concentrates on developing new methods and schemesto eventually proceed to systems which are most appropriate for the physical and chemicalquestions and applications When research on cold molecules started many scientists thought itwill be an artificial side-field of short duration But looking back the applicability of ultracoldatomic ensembles to model ideal solid state physics was not obvious Similarly the possiblerichness of superchemistry is unexplored but calculations of some systems clearly indicate thatthere is great hope of exciting results To harvest the fruits is probably more difficult becauseof the large number of degrees of freedom which we have to address by elegant experimentaland theoretical ideas and techniques not to be lost in the overwhelming routes which naturein principle has at its disposal and selects by evolution during millions of years We have torecognize the finite human lifetime Thus a thorough study of the papers provided by excellentlaboratories will be the right way of developing ideas during the short time available to us Wethank the authors for their contributions

Acknowledgments

The authors of this editorial acknowledge support from the Research and training Networkof the European Commission lsquoCold Moleculesrsquo (Contract HPRN- CT-2002-00290) and ODfrom the European Science Foundation through the network lsquoCollisions in Atom trapsrsquo

References

[1] Heinzen D J Wynar R Drummond P D and Kheruntsyan K V 2000 Superchemistry Dynamics of coupledatomic and molecular bose-einstein condensates Phys Rev Lett 84 5029

[2] httpwwwlacu-psudfrcoldmolecules[3] httpphysicsopenacukcats[4] httpquantendynamikphysikuni-freiburgdeles houches 2006[5] Chu S 1998 Nobel lecture the manipulation of neutral particles Rev Mod Phys 70 685[6] Cohen-Tannoudji C N 1998 Nobel lecture manipulating atoms with photons Rev Mod Phys 70 707[7] Phillips W D 1998 Nobel lecture laser cooling and trapping of neutral atoms Rev Mod Phys 70 721[8] Cornell E A and Wieman C E 2002 Nobel lecture BosendashEinstein condensation in a dilute gas the first 70 years

and some recent experiments Rev Mod Phys 74 875[9] Ketterle W 2002 Nobel lecture when atoms behaves as waves BosendashEinstein condensation and the atom laser

Rev Mod Phys 74 1131[10] Weiner J Bagnato V S Zilio S C and Julienne P S 1999 Experiments and theory in cold and ultracold collisions

Rev Mod Phys 71 1

Introductory Review

[11] Schloder U Engler H Schunemann U Grimmand R and Weidemuller M 1999 Cold inelastic collisions betweenlithium and cesium in a two- species magneto-optical trap Eur Phys J D 7 331

[12] Shaffer J P Chalupczak W and Bigelow N P 1999 Highly-excited states of ultracold molecules photoassociativespectroscopy of Na2 Phys Rev Lett 83 3621

[13] Mancini M W Telles G D Caires A R L Bagnato V S and Marcassa L G 2004 Observation of ultracoldground-state heteronuclear molecules Phys Rev Lett 92 133203

[14] Thorsheim H R Weiner J and Julienne P S 1987 Laser-induced photoassociation of ultracold sodium atomsPhys Rev Lett 58 2420

[15] Lett P D Helmerson K Philips W D Ratliff L P RolstonS L and Wagshul M E 1993 Spectroscopy of Na2 byphotoassociation of laser-cooled Na Phys Rev Lett 71 2200

[16] Miller J D Cline R A and Heinzen D J 1993 Photoassociation spectrum of ultracold Rb atoms Phys Rev Lett71 2204

[17] Azizi S Aymar M and Dulieu O 2004 Prospects for the formation of ultracold ground state polar moleculesfrom mixed alkali atom pair Eur Phys J D 31 195

[18] Lett P D Julienne P S and Philips W D 1995 Photoassociative spectroscopy of laser-cooled atoms Annu RevPhys Chem 46 423

[19] Stwalley W C and Wang H 1999 Photoassociation of ultracold atoms a new spectroscopic technique J MolSpectrosc 195 194

[20] Jones K M Tiesinga E Lett P D and Julienne P S 2006 Ultracold photoassociation spectroscopy Long-rangemolecules and atomic scattering Rep Prog Phys 78 483

[21] Stwalley W C Uang Y H and Pichler G 1978 Pure long-range molecules Phys Rev Lett 41 1164[22] Bohn J L and Julienne P S 1996 Semianalytic treatment of two-color photoassociation spectroscopy and control

of cold atoms Phys Rev A 54 R4637[23] Bohn J L and Julienne P S 1999 Semianalytic theory of laser-assisted resonant cold collisions Phys Rev A 60

414[24] Fioretti A Comparat D Crubellier A Dulieu O Masnou-Seeuws F and Pillet P 1998 Formation of Cs2 cold

molecules through photoassociation Phys Rev Lett 80 4402[25] C Gabbanini A Fioretti A Lucchesini S Gozzini and M Mazzoni 2000 Cold rubidium molecules formed in a

magneto-optical trap Phys Rev Lett 84 2814[26] Dion C M Drag C Dulieu O Laburthe Tolra B Masnou-Seeuws F and Pillet P 2001 Resonant coupling in the

formation of ultracold ground state molecules via photoassociation Phys Rev Lett 86 2253[27] Fatemi F K Jones K M Lett P D and Tiesinga E 2002 Ultracold ground-state molecule production in sodium

Phys Rev A 66 053401[28] Nikolov A N Eyler E E Wang X T Li J Wang H Stwalley W C and Gould P L 1999 Observation of ultracold

ground state potassium molecules Phys Rev Lett 82 703[29] Nikolov A N Enscher J R Eyler E E Wang H Stwalley W C and Gould P L 2000 Efficient production of ground

state potassium molecules at sub-mK temperatures by two-step photoassociation Phys Rev Lett 84 246[30] Band Y B and Julienne P S 1995 Ultracold molecule production by laser-cooled atom photoassociation Phys

Rev A 51 R4317[31] Lozeille J Fioretti A Gabbanini C Huang Y Pechkis H K Wang D Gould P L Eyler E E Stwalley W C

Aymar M and Dulieu O 2006 Detection of cold Rb2 triplet state molecules by two-photon ionization EurPhys J D 39

[32] Damski B Santos L Tiemann E Lewenstein M Kotochigova S Julienne P and Zoller P 2003 Creation of adipolar superfluid in optical lattices Phys Rev Lett 90 110401

[33] DeMille D 2002 Quantum computation with trapped polar molecules Phys Rev Lett 88 067901[34] Kallush S Segev B and Cote R 2005 Evanescent-wave mirror for ultracold diatomic polar molecules Phys Rev

Lett 95 163005[35] Sandars P G H 1967 Measurability of th eproton electric dipole moment Phys Rev Lett 19 1396[36] Hudson J J Sauer B E Tarbutt M R and Hinds E A 2002 Measurement of the electron electric dipole moment

using YbF molecules Phys Rev Lett 89 023003[37] Kawall D Bay F Bickman S Jiang Y and DeMille D 2004 Precision ZeemanndashStark spectroscopy of the

metastable a(1)3σ + state of PbO Phys Rev Lett 92 133007[38] Doyle J Friedrich B Krems R V and Masnou-Seeuws F 2004 Special issue on ultracold polar molecules 2004

Eur Phys J D 31 149 and following papers[39] Wang H Gould P L and Stwalley W C 1998 Fine-structure predissociation of ultracold photoassociated39K2

molecules observed by fragmentation spectroscopy Phys Rev Lett 80 476[40] Kerman A J Sage J M Sainis S Bergeman T and DeMille D 2004 Production and state-selective detection of

ultracold rbcs molecules Phys Rev Lett 92 153001

Introductory Review

[41] Wang D Qi J Stone M F Nikolayeva O Wang H Hattaway B Gensemer S D Gould P L Eyler E E andStwalley W C 2004 Photoassociative production and trapping of ultracold krb molecules Phys Rev Lett 93243005

[42] Sage J M Sainis S Bergeman T and DeMille D 2005 Optical production of ultracold polar molecules PhysRev Lett 94 203001

[43] Wang D Qi J Stone M F Nikolayeva O Hattaway B Gensemer S D Wang H Zemke W T Gould P LEyler E E and Stwalley W C 2004 The photoassociative spectroscopy photoassociative molecule formationand trapping of ultracold 39K85Rb Eur Phys J D 31 165

[44] Orozco L 2006 private communication[45] Sprouse G D Fliller R P Grossman J S Orozco L A and Pearson M R 2002 Traps for neutral radioactive atoms

Nucl Phys A 701 597[46] Abraham E R I McAlexander W I Sackett C A and Hulet R G 1995 Spectroscopic determination of the s-wave

scattering length of lithium Phys Rev Lett 74 1315[47] Tsai C C Freeland R S Vogels J M Boesten H M J M Gardner J R Heinzen D J and Verhaar B J 1997

Two-color photoassociation spectroscopy of ground state Rb2 Phys Rev Lett 79 1245[48] Laburthe Tolra B Drag C and Pillet P 2001 Formation of cold cesium molecules through stimulated raman

photoassociation Phys Rev A 64 R61401[49] Machholm M Giusti-Suzor A and Mies F H 1994 Photoassociation of atoms in ultracold collisions probed

bywave-packet dynamics Phys Rev A 50 5025[50] Judson R and Rabitz H 1992 Teaching lasers to control molecules Phys Rev Lett 68 1500[51] Vala J Dulieu O Masnou-Seeuws F and Kosloff R 2001 Coherent control of cold molecule formation through

photoassociation using a chirped pulsed laser field Phys Rev A 63 013412[52] Koch C P Luc-Koenig E and Masnou-Seeuws F 2006 Making ultracold molecules in a two color pump-dump

photoassociation scheme using chirped pulses Phys Rev A 73 033408[53] Wright M J Gensemer S D Vala J Kosloff R and Gould P L 2005 Control of ultracold collisions with frequency-

chirped light Phys Rev Lett 95 063001[54] Brown B L Dicks A J and Walmsley I A 2006 Coherent control of ultracold molecule dynamics in a magneto-

optical trap by use of chirped femtosecond laser pulses Phys Rev Lett 96 173002[55] Salzmann W Poschinger U Wester R Weidemuller M Merli A Weber S M Sauer F Plewicki M Weise F

Esparza A M Woste L and Lindinger A 2006 Coherent control with shaped femtosecond laser pulses appliedto ultracold molecules Phys Rev A 73 023414

[56] Weinstein J D deCarvalho R Kim J Patterson D Friedrich B and Doyle J M 1998 Magnetic trapping of atomicchromium Phys Rev A 57 R3173

[57] Bethlem H L and Meijer G 2003 Production and application of translationally cold molecules Int Rev PhysChem 22 73

[58] Rangwala S A Junglen T Rieger T Pinkse P W H and Rempe G 2003 Contnuous source of translationally colddipolar molecules Phys Rev A 64 043406

[59] Tarbutt M R Bethlem H L Hudson J J Ryabov V L Ryzhov V A Sauer B E Meijer G and Hinds E A 2004Slowing heavy ground-state molecules using an alternating gradient decelerator Phys Rev Lett 92 173002

[60] Staanum P Kraft S D Lange J Wester R and Weidemuller M 2006 Experimental investigation of ultracoldatom-molecule collisions Phys Rev Lett 96 023201

[61] Zahzam N Vogt T Mudrich M Comparat D and Pillet P 2006 Atomndashmolecule collisions in an optically trappedgas Phys Rev Lett 96 023202

Introductory Review

as much as 75 of the incoming one This might open research to much wider classes ofmolecules The paper by JUNG et al brings applications of the decelerated SO2 molecule intoview They measured the electric dipole moment of an excited state which they propose to usein a photodissociation experiment to produce cold radicals such as SO and O and the electricdipole moment could allow tuning of the photodissociation and thus the kinetic energy of thefragments

New recipes

After the fast development of different cooling recipes we need recipes and concepts forapplications With ultracold atomic ensembles in optical lattices one is able to model differentvariants of the BosendashHubbard system eg the transition from the Mott-insulator to thesuperfluid phase and the creation of ultracold dipolar molecular gases will add an importantnew ingredient by the anisotropy of the long range interaction to the richness of possible phasetransitions

But ultracold molecules opened the presently emerging field of ultracold chemistry witheither quantum effects (superchemistry with degenerate gases) and control of interactions withexternal fields (laser or static magnetic and electric fields) being new recipes in chemistry Firstexperimental observations of ultracold atomndashmolecule reactions have been reported [60 61]studying the lifetime of Cs2 ensembles embedded in Cs ensembles More theoretical modelsare required with which good experimental systems can be proposed or guided in future Thetheoretical work by WECK AND BALAKRISHNAN gives a general introduction to the physicalsituation of atomndashmolecule reactions at threshold and emphasizes the importance of theappearance of resonances otherwise low reaction rates are normally expected In the paper byYANG et al the link between ultracold collisions of atoms and molecules for quenching and theexperiments of buffer gas cooling mentioned above is presented for the example of rotationalquenching of CO through collision with H He or H2 in a wide range of energies from 10 μKto about 10 K as needed in the different stages of cooling processes The paper by GONZALEZ-SANCHEZ et al reports on similar aspects on the even simpler system OHminus in its ground state1+ with He atoms in 1S0 to be as complete as possible in the theoretical modelling Allthese calculations give rate constants which are of a magnitude to be observable in upcomingexperiments ultracold chemistry is probably richer than is thought by many chemists

Ion crystals but not traditional salt of the kitchen

Ion reactions play an important role in chemistry and for the cold regime the kitchen withinthe cold interstellar clouds comes immediately into view as a highly diluted system Thustrapped molecular ions certainly belong to a representation of the status of research of coldmolecules and their applications The group at Dusseldorf demonstrates in the paper by ROTH

et al the general applicability of sympathetic cooling of reaction partners like noble gas ionsor N+

2 and O+2 by laser cooled Be+ ions to produce a large variety of new cold ion samples eg

H+3 N2H+ etc These may serve for high-precision measurements for fundamental physical

questions like the time variation of fundamental constants or for studies of reaction processesrelevant in interstellar chemistry

Because of the strong Coulomb interaction between ions they appear regularly as Coulombcrystals if sufficiently cooled which gives great advantages because the ions within a crystalare well localized and thus addressable for particle by particle interaction and single particledetection To be quantum state specific in such studies the preparation of the initial physical

Introductory Review

condition is of fundamental importance This problem is addressed by two papers of theDrewsen group at Arhus In the first one by VOGELIUS et al they theoretically study the case ofrotational cooling of a polar molecule (here an ion which has an electric dipole moment withreference to its centre of mass) by optical coupling of the collective modes of a two-ion systemAs an example they describe the situation of MgH+ which is easily sympathetically cooled byCa+ In a second paper by the same authors they explore the possibility for probabilistic statepreparation in single molecular ions by projection measurements

Conclusion

Cold molecules offer a fantastic opportunity to develop strongly interdisciplinary research Twodomains are presently in view condensed matter physics with strongly correlated particlesand chemistry and superchemistry with the clear appearance of quantum effects in reactionprocesses This special issue on lsquoCold Moleculesrsquo gives insight into present lsquocookingrsquo inthe different laboratories which still concentrates on developing new methods and schemesto eventually proceed to systems which are most appropriate for the physical and chemicalquestions and applications When research on cold molecules started many scientists thought itwill be an artificial side-field of short duration But looking back the applicability of ultracoldatomic ensembles to model ideal solid state physics was not obvious Similarly the possiblerichness of superchemistry is unexplored but calculations of some systems clearly indicate thatthere is great hope of exciting results To harvest the fruits is probably more difficult becauseof the large number of degrees of freedom which we have to address by elegant experimentaland theoretical ideas and techniques not to be lost in the overwhelming routes which naturein principle has at its disposal and selects by evolution during millions of years We have torecognize the finite human lifetime Thus a thorough study of the papers provided by excellentlaboratories will be the right way of developing ideas during the short time available to us Wethank the authors for their contributions

Acknowledgments

The authors of this editorial acknowledge support from the Research and training Networkof the European Commission lsquoCold Moleculesrsquo (Contract HPRN- CT-2002-00290) and ODfrom the European Science Foundation through the network lsquoCollisions in Atom trapsrsquo

References

[1] Heinzen D J Wynar R Drummond P D and Kheruntsyan K V 2000 Superchemistry Dynamics of coupledatomic and molecular bose-einstein condensates Phys Rev Lett 84 5029

[2] httpwwwlacu-psudfrcoldmolecules[3] httpphysicsopenacukcats[4] httpquantendynamikphysikuni-freiburgdeles houches 2006[5] Chu S 1998 Nobel lecture the manipulation of neutral particles Rev Mod Phys 70 685[6] Cohen-Tannoudji C N 1998 Nobel lecture manipulating atoms with photons Rev Mod Phys 70 707[7] Phillips W D 1998 Nobel lecture laser cooling and trapping of neutral atoms Rev Mod Phys 70 721[8] Cornell E A and Wieman C E 2002 Nobel lecture BosendashEinstein condensation in a dilute gas the first 70 years

and some recent experiments Rev Mod Phys 74 875[9] Ketterle W 2002 Nobel lecture when atoms behaves as waves BosendashEinstein condensation and the atom laser

Rev Mod Phys 74 1131[10] Weiner J Bagnato V S Zilio S C and Julienne P S 1999 Experiments and theory in cold and ultracold collisions

Rev Mod Phys 71 1

Introductory Review

[11] Schloder U Engler H Schunemann U Grimmand R and Weidemuller M 1999 Cold inelastic collisions betweenlithium and cesium in a two- species magneto-optical trap Eur Phys J D 7 331

[12] Shaffer J P Chalupczak W and Bigelow N P 1999 Highly-excited states of ultracold molecules photoassociativespectroscopy of Na2 Phys Rev Lett 83 3621

[13] Mancini M W Telles G D Caires A R L Bagnato V S and Marcassa L G 2004 Observation of ultracoldground-state heteronuclear molecules Phys Rev Lett 92 133203

[14] Thorsheim H R Weiner J and Julienne P S 1987 Laser-induced photoassociation of ultracold sodium atomsPhys Rev Lett 58 2420

[15] Lett P D Helmerson K Philips W D Ratliff L P RolstonS L and Wagshul M E 1993 Spectroscopy of Na2 byphotoassociation of laser-cooled Na Phys Rev Lett 71 2200

[16] Miller J D Cline R A and Heinzen D J 1993 Photoassociation spectrum of ultracold Rb atoms Phys Rev Lett71 2204

[17] Azizi S Aymar M and Dulieu O 2004 Prospects for the formation of ultracold ground state polar moleculesfrom mixed alkali atom pair Eur Phys J D 31 195

[18] Lett P D Julienne P S and Philips W D 1995 Photoassociative spectroscopy of laser-cooled atoms Annu RevPhys Chem 46 423

[19] Stwalley W C and Wang H 1999 Photoassociation of ultracold atoms a new spectroscopic technique J MolSpectrosc 195 194

[20] Jones K M Tiesinga E Lett P D and Julienne P S 2006 Ultracold photoassociation spectroscopy Long-rangemolecules and atomic scattering Rep Prog Phys 78 483

[21] Stwalley W C Uang Y H and Pichler G 1978 Pure long-range molecules Phys Rev Lett 41 1164[22] Bohn J L and Julienne P S 1996 Semianalytic treatment of two-color photoassociation spectroscopy and control

of cold atoms Phys Rev A 54 R4637[23] Bohn J L and Julienne P S 1999 Semianalytic theory of laser-assisted resonant cold collisions Phys Rev A 60

414[24] Fioretti A Comparat D Crubellier A Dulieu O Masnou-Seeuws F and Pillet P 1998 Formation of Cs2 cold

molecules through photoassociation Phys Rev Lett 80 4402[25] C Gabbanini A Fioretti A Lucchesini S Gozzini and M Mazzoni 2000 Cold rubidium molecules formed in a

magneto-optical trap Phys Rev Lett 84 2814[26] Dion C M Drag C Dulieu O Laburthe Tolra B Masnou-Seeuws F and Pillet P 2001 Resonant coupling in the

formation of ultracold ground state molecules via photoassociation Phys Rev Lett 86 2253[27] Fatemi F K Jones K M Lett P D and Tiesinga E 2002 Ultracold ground-state molecule production in sodium

Phys Rev A 66 053401[28] Nikolov A N Eyler E E Wang X T Li J Wang H Stwalley W C and Gould P L 1999 Observation of ultracold

ground state potassium molecules Phys Rev Lett 82 703[29] Nikolov A N Enscher J R Eyler E E Wang H Stwalley W C and Gould P L 2000 Efficient production of ground

state potassium molecules at sub-mK temperatures by two-step photoassociation Phys Rev Lett 84 246[30] Band Y B and Julienne P S 1995 Ultracold molecule production by laser-cooled atom photoassociation Phys

Rev A 51 R4317[31] Lozeille J Fioretti A Gabbanini C Huang Y Pechkis H K Wang D Gould P L Eyler E E Stwalley W C

Aymar M and Dulieu O 2006 Detection of cold Rb2 triplet state molecules by two-photon ionization EurPhys J D 39

[32] Damski B Santos L Tiemann E Lewenstein M Kotochigova S Julienne P and Zoller P 2003 Creation of adipolar superfluid in optical lattices Phys Rev Lett 90 110401

[33] DeMille D 2002 Quantum computation with trapped polar molecules Phys Rev Lett 88 067901[34] Kallush S Segev B and Cote R 2005 Evanescent-wave mirror for ultracold diatomic polar molecules Phys Rev

Lett 95 163005[35] Sandars P G H 1967 Measurability of th eproton electric dipole moment Phys Rev Lett 19 1396[36] Hudson J J Sauer B E Tarbutt M R and Hinds E A 2002 Measurement of the electron electric dipole moment

using YbF molecules Phys Rev Lett 89 023003[37] Kawall D Bay F Bickman S Jiang Y and DeMille D 2004 Precision ZeemanndashStark spectroscopy of the

metastable a(1)3σ + state of PbO Phys Rev Lett 92 133007[38] Doyle J Friedrich B Krems R V and Masnou-Seeuws F 2004 Special issue on ultracold polar molecules 2004

Eur Phys J D 31 149 and following papers[39] Wang H Gould P L and Stwalley W C 1998 Fine-structure predissociation of ultracold photoassociated39K2

molecules observed by fragmentation spectroscopy Phys Rev Lett 80 476[40] Kerman A J Sage J M Sainis S Bergeman T and DeMille D 2004 Production and state-selective detection of

ultracold rbcs molecules Phys Rev Lett 92 153001

Introductory Review

[41] Wang D Qi J Stone M F Nikolayeva O Wang H Hattaway B Gensemer S D Gould P L Eyler E E andStwalley W C 2004 Photoassociative production and trapping of ultracold krb molecules Phys Rev Lett 93243005

[42] Sage J M Sainis S Bergeman T and DeMille D 2005 Optical production of ultracold polar molecules PhysRev Lett 94 203001

[43] Wang D Qi J Stone M F Nikolayeva O Hattaway B Gensemer S D Wang H Zemke W T Gould P LEyler E E and Stwalley W C 2004 The photoassociative spectroscopy photoassociative molecule formationand trapping of ultracold 39K85Rb Eur Phys J D 31 165

[44] Orozco L 2006 private communication[45] Sprouse G D Fliller R P Grossman J S Orozco L A and Pearson M R 2002 Traps for neutral radioactive atoms

Nucl Phys A 701 597[46] Abraham E R I McAlexander W I Sackett C A and Hulet R G 1995 Spectroscopic determination of the s-wave

scattering length of lithium Phys Rev Lett 74 1315[47] Tsai C C Freeland R S Vogels J M Boesten H M J M Gardner J R Heinzen D J and Verhaar B J 1997

Two-color photoassociation spectroscopy of ground state Rb2 Phys Rev Lett 79 1245[48] Laburthe Tolra B Drag C and Pillet P 2001 Formation of cold cesium molecules through stimulated raman

photoassociation Phys Rev A 64 R61401[49] Machholm M Giusti-Suzor A and Mies F H 1994 Photoassociation of atoms in ultracold collisions probed

bywave-packet dynamics Phys Rev A 50 5025[50] Judson R and Rabitz H 1992 Teaching lasers to control molecules Phys Rev Lett 68 1500[51] Vala J Dulieu O Masnou-Seeuws F and Kosloff R 2001 Coherent control of cold molecule formation through

photoassociation using a chirped pulsed laser field Phys Rev A 63 013412[52] Koch C P Luc-Koenig E and Masnou-Seeuws F 2006 Making ultracold molecules in a two color pump-dump

photoassociation scheme using chirped pulses Phys Rev A 73 033408[53] Wright M J Gensemer S D Vala J Kosloff R and Gould P L 2005 Control of ultracold collisions with frequency-

chirped light Phys Rev Lett 95 063001[54] Brown B L Dicks A J and Walmsley I A 2006 Coherent control of ultracold molecule dynamics in a magneto-

optical trap by use of chirped femtosecond laser pulses Phys Rev Lett 96 173002[55] Salzmann W Poschinger U Wester R Weidemuller M Merli A Weber S M Sauer F Plewicki M Weise F

Esparza A M Woste L and Lindinger A 2006 Coherent control with shaped femtosecond laser pulses appliedto ultracold molecules Phys Rev A 73 023414

[56] Weinstein J D deCarvalho R Kim J Patterson D Friedrich B and Doyle J M 1998 Magnetic trapping of atomicchromium Phys Rev A 57 R3173

[57] Bethlem H L and Meijer G 2003 Production and application of translationally cold molecules Int Rev PhysChem 22 73

[58] Rangwala S A Junglen T Rieger T Pinkse P W H and Rempe G 2003 Contnuous source of translationally colddipolar molecules Phys Rev A 64 043406

[59] Tarbutt M R Bethlem H L Hudson J J Ryabov V L Ryzhov V A Sauer B E Meijer G and Hinds E A 2004Slowing heavy ground-state molecules using an alternating gradient decelerator Phys Rev Lett 92 173002

[60] Staanum P Kraft S D Lange J Wester R and Weidemuller M 2006 Experimental investigation of ultracoldatom-molecule collisions Phys Rev Lett 96 023201

[61] Zahzam N Vogt T Mudrich M Comparat D and Pillet P 2006 Atomndashmolecule collisions in an optically trappedgas Phys Rev Lett 96 023202

Introductory Review

condition is of fundamental importance This problem is addressed by two papers of theDrewsen group at Arhus In the first one by VOGELIUS et al they theoretically study the case ofrotational cooling of a polar molecule (here an ion which has an electric dipole moment withreference to its centre of mass) by optical coupling of the collective modes of a two-ion systemAs an example they describe the situation of MgH+ which is easily sympathetically cooled byCa+ In a second paper by the same authors they explore the possibility for probabilistic statepreparation in single molecular ions by projection measurements

Conclusion

Cold molecules offer a fantastic opportunity to develop strongly interdisciplinary research Twodomains are presently in view condensed matter physics with strongly correlated particlesand chemistry and superchemistry with the clear appearance of quantum effects in reactionprocesses This special issue on lsquoCold Moleculesrsquo gives insight into present lsquocookingrsquo inthe different laboratories which still concentrates on developing new methods and schemesto eventually proceed to systems which are most appropriate for the physical and chemicalquestions and applications When research on cold molecules started many scientists thought itwill be an artificial side-field of short duration But looking back the applicability of ultracoldatomic ensembles to model ideal solid state physics was not obvious Similarly the possiblerichness of superchemistry is unexplored but calculations of some systems clearly indicate thatthere is great hope of exciting results To harvest the fruits is probably more difficult becauseof the large number of degrees of freedom which we have to address by elegant experimentaland theoretical ideas and techniques not to be lost in the overwhelming routes which naturein principle has at its disposal and selects by evolution during millions of years We have torecognize the finite human lifetime Thus a thorough study of the papers provided by excellentlaboratories will be the right way of developing ideas during the short time available to us Wethank the authors for their contributions

Acknowledgments

The authors of this editorial acknowledge support from the Research and training Networkof the European Commission lsquoCold Moleculesrsquo (Contract HPRN- CT-2002-00290) and ODfrom the European Science Foundation through the network lsquoCollisions in Atom trapsrsquo

References

[1] Heinzen D J Wynar R Drummond P D and Kheruntsyan K V 2000 Superchemistry Dynamics of coupledatomic and molecular bose-einstein condensates Phys Rev Lett 84 5029

[2] httpwwwlacu-psudfrcoldmolecules[3] httpphysicsopenacukcats[4] httpquantendynamikphysikuni-freiburgdeles houches 2006[5] Chu S 1998 Nobel lecture the manipulation of neutral particles Rev Mod Phys 70 685[6] Cohen-Tannoudji C N 1998 Nobel lecture manipulating atoms with photons Rev Mod Phys 70 707[7] Phillips W D 1998 Nobel lecture laser cooling and trapping of neutral atoms Rev Mod Phys 70 721[8] Cornell E A and Wieman C E 2002 Nobel lecture BosendashEinstein condensation in a dilute gas the first 70 years

and some recent experiments Rev Mod Phys 74 875[9] Ketterle W 2002 Nobel lecture when atoms behaves as waves BosendashEinstein condensation and the atom laser

Rev Mod Phys 74 1131[10] Weiner J Bagnato V S Zilio S C and Julienne P S 1999 Experiments and theory in cold and ultracold collisions

Rev Mod Phys 71 1

Introductory Review

[11] Schloder U Engler H Schunemann U Grimmand R and Weidemuller M 1999 Cold inelastic collisions betweenlithium and cesium in a two- species magneto-optical trap Eur Phys J D 7 331

[12] Shaffer J P Chalupczak W and Bigelow N P 1999 Highly-excited states of ultracold molecules photoassociativespectroscopy of Na2 Phys Rev Lett 83 3621

[13] Mancini M W Telles G D Caires A R L Bagnato V S and Marcassa L G 2004 Observation of ultracoldground-state heteronuclear molecules Phys Rev Lett 92 133203

[14] Thorsheim H R Weiner J and Julienne P S 1987 Laser-induced photoassociation of ultracold sodium atomsPhys Rev Lett 58 2420

[15] Lett P D Helmerson K Philips W D Ratliff L P RolstonS L and Wagshul M E 1993 Spectroscopy of Na2 byphotoassociation of laser-cooled Na Phys Rev Lett 71 2200

[16] Miller J D Cline R A and Heinzen D J 1993 Photoassociation spectrum of ultracold Rb atoms Phys Rev Lett71 2204

[17] Azizi S Aymar M and Dulieu O 2004 Prospects for the formation of ultracold ground state polar moleculesfrom mixed alkali atom pair Eur Phys J D 31 195

[18] Lett P D Julienne P S and Philips W D 1995 Photoassociative spectroscopy of laser-cooled atoms Annu RevPhys Chem 46 423

[19] Stwalley W C and Wang H 1999 Photoassociation of ultracold atoms a new spectroscopic technique J MolSpectrosc 195 194

[20] Jones K M Tiesinga E Lett P D and Julienne P S 2006 Ultracold photoassociation spectroscopy Long-rangemolecules and atomic scattering Rep Prog Phys 78 483

[21] Stwalley W C Uang Y H and Pichler G 1978 Pure long-range molecules Phys Rev Lett 41 1164[22] Bohn J L and Julienne P S 1996 Semianalytic treatment of two-color photoassociation spectroscopy and control

of cold atoms Phys Rev A 54 R4637[23] Bohn J L and Julienne P S 1999 Semianalytic theory of laser-assisted resonant cold collisions Phys Rev A 60

414[24] Fioretti A Comparat D Crubellier A Dulieu O Masnou-Seeuws F and Pillet P 1998 Formation of Cs2 cold

molecules through photoassociation Phys Rev Lett 80 4402[25] C Gabbanini A Fioretti A Lucchesini S Gozzini and M Mazzoni 2000 Cold rubidium molecules formed in a

magneto-optical trap Phys Rev Lett 84 2814[26] Dion C M Drag C Dulieu O Laburthe Tolra B Masnou-Seeuws F and Pillet P 2001 Resonant coupling in the

formation of ultracold ground state molecules via photoassociation Phys Rev Lett 86 2253[27] Fatemi F K Jones K M Lett P D and Tiesinga E 2002 Ultracold ground-state molecule production in sodium

Phys Rev A 66 053401[28] Nikolov A N Eyler E E Wang X T Li J Wang H Stwalley W C and Gould P L 1999 Observation of ultracold

ground state potassium molecules Phys Rev Lett 82 703[29] Nikolov A N Enscher J R Eyler E E Wang H Stwalley W C and Gould P L 2000 Efficient production of ground

state potassium molecules at sub-mK temperatures by two-step photoassociation Phys Rev Lett 84 246[30] Band Y B and Julienne P S 1995 Ultracold molecule production by laser-cooled atom photoassociation Phys

Rev A 51 R4317[31] Lozeille J Fioretti A Gabbanini C Huang Y Pechkis H K Wang D Gould P L Eyler E E Stwalley W C

Aymar M and Dulieu O 2006 Detection of cold Rb2 triplet state molecules by two-photon ionization EurPhys J D 39

[32] Damski B Santos L Tiemann E Lewenstein M Kotochigova S Julienne P and Zoller P 2003 Creation of adipolar superfluid in optical lattices Phys Rev Lett 90 110401

[33] DeMille D 2002 Quantum computation with trapped polar molecules Phys Rev Lett 88 067901[34] Kallush S Segev B and Cote R 2005 Evanescent-wave mirror for ultracold diatomic polar molecules Phys Rev

Lett 95 163005[35] Sandars P G H 1967 Measurability of th eproton electric dipole moment Phys Rev Lett 19 1396[36] Hudson J J Sauer B E Tarbutt M R and Hinds E A 2002 Measurement of the electron electric dipole moment

using YbF molecules Phys Rev Lett 89 023003[37] Kawall D Bay F Bickman S Jiang Y and DeMille D 2004 Precision ZeemanndashStark spectroscopy of the

metastable a(1)3σ + state of PbO Phys Rev Lett 92 133007[38] Doyle J Friedrich B Krems R V and Masnou-Seeuws F 2004 Special issue on ultracold polar molecules 2004

Eur Phys J D 31 149 and following papers[39] Wang H Gould P L and Stwalley W C 1998 Fine-structure predissociation of ultracold photoassociated39K2

molecules observed by fragmentation spectroscopy Phys Rev Lett 80 476[40] Kerman A J Sage J M Sainis S Bergeman T and DeMille D 2004 Production and state-selective detection of

ultracold rbcs molecules Phys Rev Lett 92 153001

Introductory Review

[41] Wang D Qi J Stone M F Nikolayeva O Wang H Hattaway B Gensemer S D Gould P L Eyler E E andStwalley W C 2004 Photoassociative production and trapping of ultracold krb molecules Phys Rev Lett 93243005

[42] Sage J M Sainis S Bergeman T and DeMille D 2005 Optical production of ultracold polar molecules PhysRev Lett 94 203001

[43] Wang D Qi J Stone M F Nikolayeva O Hattaway B Gensemer S D Wang H Zemke W T Gould P LEyler E E and Stwalley W C 2004 The photoassociative spectroscopy photoassociative molecule formationand trapping of ultracold 39K85Rb Eur Phys J D 31 165

[44] Orozco L 2006 private communication[45] Sprouse G D Fliller R P Grossman J S Orozco L A and Pearson M R 2002 Traps for neutral radioactive atoms

Nucl Phys A 701 597[46] Abraham E R I McAlexander W I Sackett C A and Hulet R G 1995 Spectroscopic determination of the s-wave

scattering length of lithium Phys Rev Lett 74 1315[47] Tsai C C Freeland R S Vogels J M Boesten H M J M Gardner J R Heinzen D J and Verhaar B J 1997

Two-color photoassociation spectroscopy of ground state Rb2 Phys Rev Lett 79 1245[48] Laburthe Tolra B Drag C and Pillet P 2001 Formation of cold cesium molecules through stimulated raman

photoassociation Phys Rev A 64 R61401[49] Machholm M Giusti-Suzor A and Mies F H 1994 Photoassociation of atoms in ultracold collisions probed

bywave-packet dynamics Phys Rev A 50 5025[50] Judson R and Rabitz H 1992 Teaching lasers to control molecules Phys Rev Lett 68 1500[51] Vala J Dulieu O Masnou-Seeuws F and Kosloff R 2001 Coherent control of cold molecule formation through

photoassociation using a chirped pulsed laser field Phys Rev A 63 013412[52] Koch C P Luc-Koenig E and Masnou-Seeuws F 2006 Making ultracold molecules in a two color pump-dump

photoassociation scheme using chirped pulses Phys Rev A 73 033408[53] Wright M J Gensemer S D Vala J Kosloff R and Gould P L 2005 Control of ultracold collisions with frequency-

chirped light Phys Rev Lett 95 063001[54] Brown B L Dicks A J and Walmsley I A 2006 Coherent control of ultracold molecule dynamics in a magneto-

optical trap by use of chirped femtosecond laser pulses Phys Rev Lett 96 173002[55] Salzmann W Poschinger U Wester R Weidemuller M Merli A Weber S M Sauer F Plewicki M Weise F

Esparza A M Woste L and Lindinger A 2006 Coherent control with shaped femtosecond laser pulses appliedto ultracold molecules Phys Rev A 73 023414

[56] Weinstein J D deCarvalho R Kim J Patterson D Friedrich B and Doyle J M 1998 Magnetic trapping of atomicchromium Phys Rev A 57 R3173

[57] Bethlem H L and Meijer G 2003 Production and application of translationally cold molecules Int Rev PhysChem 22 73

[58] Rangwala S A Junglen T Rieger T Pinkse P W H and Rempe G 2003 Contnuous source of translationally colddipolar molecules Phys Rev A 64 043406

[59] Tarbutt M R Bethlem H L Hudson J J Ryabov V L Ryzhov V A Sauer B E Meijer G and Hinds E A 2004Slowing heavy ground-state molecules using an alternating gradient decelerator Phys Rev Lett 92 173002

[60] Staanum P Kraft S D Lange J Wester R and Weidemuller M 2006 Experimental investigation of ultracoldatom-molecule collisions Phys Rev Lett 96 023201

[61] Zahzam N Vogt T Mudrich M Comparat D and Pillet P 2006 Atomndashmolecule collisions in an optically trappedgas Phys Rev Lett 96 023202

Introductory Review

[11] Schloder U Engler H Schunemann U Grimmand R and Weidemuller M 1999 Cold inelastic collisions betweenlithium and cesium in a two- species magneto-optical trap Eur Phys J D 7 331

[12] Shaffer J P Chalupczak W and Bigelow N P 1999 Highly-excited states of ultracold molecules photoassociativespectroscopy of Na2 Phys Rev Lett 83 3621

[13] Mancini M W Telles G D Caires A R L Bagnato V S and Marcassa L G 2004 Observation of ultracoldground-state heteronuclear molecules Phys Rev Lett 92 133203

[14] Thorsheim H R Weiner J and Julienne P S 1987 Laser-induced photoassociation of ultracold sodium atomsPhys Rev Lett 58 2420

[15] Lett P D Helmerson K Philips W D Ratliff L P RolstonS L and Wagshul M E 1993 Spectroscopy of Na2 byphotoassociation of laser-cooled Na Phys Rev Lett 71 2200

[16] Miller J D Cline R A and Heinzen D J 1993 Photoassociation spectrum of ultracold Rb atoms Phys Rev Lett71 2204

[17] Azizi S Aymar M and Dulieu O 2004 Prospects for the formation of ultracold ground state polar moleculesfrom mixed alkali atom pair Eur Phys J D 31 195

[18] Lett P D Julienne P S and Philips W D 1995 Photoassociative spectroscopy of laser-cooled atoms Annu RevPhys Chem 46 423

[19] Stwalley W C and Wang H 1999 Photoassociation of ultracold atoms a new spectroscopic technique J MolSpectrosc 195 194

[20] Jones K M Tiesinga E Lett P D and Julienne P S 2006 Ultracold photoassociation spectroscopy Long-rangemolecules and atomic scattering Rep Prog Phys 78 483

[21] Stwalley W C Uang Y H and Pichler G 1978 Pure long-range molecules Phys Rev Lett 41 1164[22] Bohn J L and Julienne P S 1996 Semianalytic treatment of two-color photoassociation spectroscopy and control

of cold atoms Phys Rev A 54 R4637[23] Bohn J L and Julienne P S 1999 Semianalytic theory of laser-assisted resonant cold collisions Phys Rev A 60

414[24] Fioretti A Comparat D Crubellier A Dulieu O Masnou-Seeuws F and Pillet P 1998 Formation of Cs2 cold

molecules through photoassociation Phys Rev Lett 80 4402[25] C Gabbanini A Fioretti A Lucchesini S Gozzini and M Mazzoni 2000 Cold rubidium molecules formed in a

magneto-optical trap Phys Rev Lett 84 2814[26] Dion C M Drag C Dulieu O Laburthe Tolra B Masnou-Seeuws F and Pillet P 2001 Resonant coupling in the

formation of ultracold ground state molecules via photoassociation Phys Rev Lett 86 2253[27] Fatemi F K Jones K M Lett P D and Tiesinga E 2002 Ultracold ground-state molecule production in sodium

Phys Rev A 66 053401[28] Nikolov A N Eyler E E Wang X T Li J Wang H Stwalley W C and Gould P L 1999 Observation of ultracold

ground state potassium molecules Phys Rev Lett 82 703[29] Nikolov A N Enscher J R Eyler E E Wang H Stwalley W C and Gould P L 2000 Efficient production of ground

state potassium molecules at sub-mK temperatures by two-step photoassociation Phys Rev Lett 84 246[30] Band Y B and Julienne P S 1995 Ultracold molecule production by laser-cooled atom photoassociation Phys

Rev A 51 R4317[31] Lozeille J Fioretti A Gabbanini C Huang Y Pechkis H K Wang D Gould P L Eyler E E Stwalley W C

Aymar M and Dulieu O 2006 Detection of cold Rb2 triplet state molecules by two-photon ionization EurPhys J D 39

[32] Damski B Santos L Tiemann E Lewenstein M Kotochigova S Julienne P and Zoller P 2003 Creation of adipolar superfluid in optical lattices Phys Rev Lett 90 110401

[33] DeMille D 2002 Quantum computation with trapped polar molecules Phys Rev Lett 88 067901[34] Kallush S Segev B and Cote R 2005 Evanescent-wave mirror for ultracold diatomic polar molecules Phys Rev

Lett 95 163005[35] Sandars P G H 1967 Measurability of th eproton electric dipole moment Phys Rev Lett 19 1396[36] Hudson J J Sauer B E Tarbutt M R and Hinds E A 2002 Measurement of the electron electric dipole moment

using YbF molecules Phys Rev Lett 89 023003[37] Kawall D Bay F Bickman S Jiang Y and DeMille D 2004 Precision ZeemanndashStark spectroscopy of the

metastable a(1)3σ + state of PbO Phys Rev Lett 92 133007[38] Doyle J Friedrich B Krems R V and Masnou-Seeuws F 2004 Special issue on ultracold polar molecules 2004

Eur Phys J D 31 149 and following papers[39] Wang H Gould P L and Stwalley W C 1998 Fine-structure predissociation of ultracold photoassociated39K2

molecules observed by fragmentation spectroscopy Phys Rev Lett 80 476[40] Kerman A J Sage J M Sainis S Bergeman T and DeMille D 2004 Production and state-selective detection of

ultracold rbcs molecules Phys Rev Lett 92 153001

Introductory Review

[41] Wang D Qi J Stone M F Nikolayeva O Wang H Hattaway B Gensemer S D Gould P L Eyler E E andStwalley W C 2004 Photoassociative production and trapping of ultracold krb molecules Phys Rev Lett 93243005

[42] Sage J M Sainis S Bergeman T and DeMille D 2005 Optical production of ultracold polar molecules PhysRev Lett 94 203001

[43] Wang D Qi J Stone M F Nikolayeva O Hattaway B Gensemer S D Wang H Zemke W T Gould P LEyler E E and Stwalley W C 2004 The photoassociative spectroscopy photoassociative molecule formationand trapping of ultracold 39K85Rb Eur Phys J D 31 165

[44] Orozco L 2006 private communication[45] Sprouse G D Fliller R P Grossman J S Orozco L A and Pearson M R 2002 Traps for neutral radioactive atoms

Nucl Phys A 701 597[46] Abraham E R I McAlexander W I Sackett C A and Hulet R G 1995 Spectroscopic determination of the s-wave

scattering length of lithium Phys Rev Lett 74 1315[47] Tsai C C Freeland R S Vogels J M Boesten H M J M Gardner J R Heinzen D J and Verhaar B J 1997

Two-color photoassociation spectroscopy of ground state Rb2 Phys Rev Lett 79 1245[48] Laburthe Tolra B Drag C and Pillet P 2001 Formation of cold cesium molecules through stimulated raman

photoassociation Phys Rev A 64 R61401[49] Machholm M Giusti-Suzor A and Mies F H 1994 Photoassociation of atoms in ultracold collisions probed

bywave-packet dynamics Phys Rev A 50 5025[50] Judson R and Rabitz H 1992 Teaching lasers to control molecules Phys Rev Lett 68 1500[51] Vala J Dulieu O Masnou-Seeuws F and Kosloff R 2001 Coherent control of cold molecule formation through

photoassociation using a chirped pulsed laser field Phys Rev A 63 013412[52] Koch C P Luc-Koenig E and Masnou-Seeuws F 2006 Making ultracold molecules in a two color pump-dump

photoassociation scheme using chirped pulses Phys Rev A 73 033408[53] Wright M J Gensemer S D Vala J Kosloff R and Gould P L 2005 Control of ultracold collisions with frequency-

chirped light Phys Rev Lett 95 063001[54] Brown B L Dicks A J and Walmsley I A 2006 Coherent control of ultracold molecule dynamics in a magneto-

optical trap by use of chirped femtosecond laser pulses Phys Rev Lett 96 173002[55] Salzmann W Poschinger U Wester R Weidemuller M Merli A Weber S M Sauer F Plewicki M Weise F

Esparza A M Woste L and Lindinger A 2006 Coherent control with shaped femtosecond laser pulses appliedto ultracold molecules Phys Rev A 73 023414

[56] Weinstein J D deCarvalho R Kim J Patterson D Friedrich B and Doyle J M 1998 Magnetic trapping of atomicchromium Phys Rev A 57 R3173

[57] Bethlem H L and Meijer G 2003 Production and application of translationally cold molecules Int Rev PhysChem 22 73

[58] Rangwala S A Junglen T Rieger T Pinkse P W H and Rempe G 2003 Contnuous source of translationally colddipolar molecules Phys Rev A 64 043406

[59] Tarbutt M R Bethlem H L Hudson J J Ryabov V L Ryzhov V A Sauer B E Meijer G and Hinds E A 2004Slowing heavy ground-state molecules using an alternating gradient decelerator Phys Rev Lett 92 173002

[60] Staanum P Kraft S D Lange J Wester R and Weidemuller M 2006 Experimental investigation of ultracoldatom-molecule collisions Phys Rev Lett 96 023201

[61] Zahzam N Vogt T Mudrich M Comparat D and Pillet P 2006 Atomndashmolecule collisions in an optically trappedgas Phys Rev Lett 96 023202

Introductory Review

[41] Wang D Qi J Stone M F Nikolayeva O Wang H Hattaway B Gensemer S D Gould P L Eyler E E andStwalley W C 2004 Photoassociative production and trapping of ultracold krb molecules Phys Rev Lett 93243005

[42] Sage J M Sainis S Bergeman T and DeMille D 2005 Optical production of ultracold polar molecules PhysRev Lett 94 203001

[43] Wang D Qi J Stone M F Nikolayeva O Hattaway B Gensemer S D Wang H Zemke W T Gould P LEyler E E and Stwalley W C 2004 The photoassociative spectroscopy photoassociative molecule formationand trapping of ultracold 39K85Rb Eur Phys J D 31 165

[44] Orozco L 2006 private communication[45] Sprouse G D Fliller R P Grossman J S Orozco L A and Pearson M R 2002 Traps for neutral radioactive atoms

Nucl Phys A 701 597[46] Abraham E R I McAlexander W I Sackett C A and Hulet R G 1995 Spectroscopic determination of the s-wave

scattering length of lithium Phys Rev Lett 74 1315[47] Tsai C C Freeland R S Vogels J M Boesten H M J M Gardner J R Heinzen D J and Verhaar B J 1997

Two-color photoassociation spectroscopy of ground state Rb2 Phys Rev Lett 79 1245[48] Laburthe Tolra B Drag C and Pillet P 2001 Formation of cold cesium molecules through stimulated raman

photoassociation Phys Rev A 64 R61401[49] Machholm M Giusti-Suzor A and Mies F H 1994 Photoassociation of atoms in ultracold collisions probed

bywave-packet dynamics Phys Rev A 50 5025[50] Judson R and Rabitz H 1992 Teaching lasers to control molecules Phys Rev Lett 68 1500[51] Vala J Dulieu O Masnou-Seeuws F and Kosloff R 2001 Coherent control of cold molecule formation through

photoassociation using a chirped pulsed laser field Phys Rev A 63 013412[52] Koch C P Luc-Koenig E and Masnou-Seeuws F 2006 Making ultracold molecules in a two color pump-dump

photoassociation scheme using chirped pulses Phys Rev A 73 033408[53] Wright M J Gensemer S D Vala J Kosloff R and Gould P L 2005 Control of ultracold collisions with frequency-

chirped light Phys Rev Lett 95 063001[54] Brown B L Dicks A J and Walmsley I A 2006 Coherent control of ultracold molecule dynamics in a magneto-

optical trap by use of chirped femtosecond laser pulses Phys Rev Lett 96 173002[55] Salzmann W Poschinger U Wester R Weidemuller M Merli A Weber S M Sauer F Plewicki M Weise F

Esparza A M Woste L and Lindinger A 2006 Coherent control with shaped femtosecond laser pulses appliedto ultracold molecules Phys Rev A 73 023414

[56] Weinstein J D deCarvalho R Kim J Patterson D Friedrich B and Doyle J M 1998 Magnetic trapping of atomicchromium Phys Rev A 57 R3173

[57] Bethlem H L and Meijer G 2003 Production and application of translationally cold molecules Int Rev PhysChem 22 73

[58] Rangwala S A Junglen T Rieger T Pinkse P W H and Rempe G 2003 Contnuous source of translationally colddipolar molecules Phys Rev A 64 043406

[59] Tarbutt M R Bethlem H L Hudson J J Ryabov V L Ryzhov V A Sauer B E Meijer G and Hinds E A 2004Slowing heavy ground-state molecules using an alternating gradient decelerator Phys Rev Lett 92 173002

[60] Staanum P Kraft S D Lange J Wester R and Weidemuller M 2006 Experimental investigation of ultracoldatom-molecule collisions Phys Rev Lett 96 023201

[61] Zahzam N Vogt T Mudrich M Comparat D and Pillet P 2006 Atomndashmolecule collisions in an optically trappedgas Phys Rev Lett 96 023202