laying the foundation for a muon-laser spectroscopy · 2016-11-18 · laying the foundation for a...

Laying the foundation for a muon-laser spectroscopyK. GhandiChemistry department, Mount Allison University and TRIUMF, Sackville, Canada

I. P. ClarkCentral Laser Facility, CCLRC Rutherford Appleton Laboratory, Chilton, Didcot, Oxon., OX11 OQX, UK

J. S. LordISIS, CCLRC Rutherford Appleton Laboratory, Chilton, Didcot, Oxon., OX11 OQX, UK

Lasers for Science Facility Programme n Chemistry

141Central Laser Facility Annual Report n 2005/2006

Main contact email address [email protected]

5

AbstractIn this report we introduce two novel techniques, laser-muon spin spectroscopy (laser-µSR) where laser excitationfollows muon arrival to excite muoniated molecules andlaser pump-muon magnetic resonance probe where laserexcitation precedes muon arrival to excite moleculesreacting with muonium or muoniated free radicals. Boththese techniques employ laser excitation to produce atransient while the chemical dynamics of which are probedusing muon spin spectroscopy (µSR). By using a tunablelaser system it will be possible to measure the electronicand vibrational excitations of a muoniated centre. This willprovide an independent tool for radical and siteassignments for muon implanted samples. This reportpresents the proof of principle experiments, using time-resolved Zero-Field µSR (ZF-µSR), to observe the changeupon 532 nm laser excitation in the ZF asymmetry of theradical adduct produced by excitation of powdered rosebengal. For the first time these pump-probe experimentsextend muonium chemistry to the realm of excited states.The results enable the detection of muoniated molecules bytheir spin evolution after laser excitation; opening newopportunities to study the chemical dynamics of muoniatedintermediates in excited states.

IntroductionOf interest to many applications in transient chemistry,including free radicals are the changes between the groundand excited states. In this report we introduce a new tool,laser-muon spin spectroscopy (laser-µSR) for examiningexcited state chemistry. Laser-muon spin spectroscopy isbased on coupling the powerful techniques of pulsed laserexcitation (as pump) and µSR (as probe). This newapproach takes advantage of the lasers and the structureof intense pulsed muon beams available at the CCLRCRutherford Appleton Laboratory (RAL) in the UK. It hasapplications in radiation chemistry, free radical chemistry,and reaction dynamics, particularly studies of kineticisotope effects in the excited state at the most sensitive endof the mass scale (i.e., through studies of KIEs of Hisotopes). These initial experiments extend µSR, whichencompasses muon spin rotation/relaxation and resonance,a suite of magnetic resonance techniques that use positivemuons (µ+) to new domains. Muons which are spin 1/2particles have a mass approximately 1/9 that of H+, can beproduced 100 % spin polarized and serve as microscopicprobes of the electronic and magnetic environments. The100 % spin polarization of muons and their anisotropicdecay, combined with advanced sensitive nuclear physicscounting techniques for the detection of decay positronsfrom muon decays, makes µSR much more sensitive thanconventional magnetic resonance techniques [1].

ExperimentalThese studies were conducted using the DEVA surfacemuon beam at ISIS and a Nd:YAG laser from the EPSRCLaser Loan Pool. The laser system, which operated at 10 Hz (ca.7 ns pulse, 500 mJ at 532 nm), was triggered toeither precede or follow the arrival of each fifth muonpulse (at 50 Hz) with a set time delay. The first four inevery five muon pulses provided the information on theground state. The laser-µSR cells were made in-house. Thesizes of the muon and laser input windows on these cellswere matched to maximize the overlap between the pumpand probe beams.

Spin polarized positive muon beams are produced bynuclear accelerators such as ISIS at MeV kinetic energies(4.1 MeV at surface muon beam lines such as DEVA).After these muons enter and stop in the target, detectorsregister positrons emitted preferentially along the directionof the muon spin when the stopped muons decay. AtDEVA, positron detectors are arranged forward(upstream) and backward, relative to the initial muon spinpolarization. For each detector, data is usually collectedfor 32 µs subsequent to the muon implantation, with thenumber of positrons detected during each 16 ns time-steprecorded and collected in a histogram of counts as afunction of time. The probability of detecting the decaypositron in a given direction varies, because the muon spinprecesses in a magnetic field, either external or internal [1].Thus µSR, histograms obtained from positron detectorsmay contain oscillations in the muon decay spectrum, andthese oscillations correspond to the time dependence ofthe muon polarization. Each histogram has the form:

(1)

where N0 is the overall normalization that depends on anumber of factors, such as the solid angle of the positrondetectors and the number of stopped muons; τµ is themuon lifetime, 2.197 µs, and A(t) is the asymmetry, whichrepresents the µSR signal of interest which is similar tofree induction decay in magnetic resonance. A normalisedasymmetry, A(t), can be defined in terms of the positroncounts from muon decay in the forward NF(t) andbackward NB(t) positron detectors:

(2)

where α takes into account the differences in relativegeometry of the stopping muons and detector efficiency.

))(1()(/

0 tAeNtNt

+=− μτ

)()(

)()()(

tNtN

tNtNtA

BF

BFα

α

+

−=

)cos()exp()( iiiii twtAtA ϕλ +−Σ=

))(1()(/

0 tAeNtNt

+=− μτ

)()(

)()()(

tNtN

tNtNtA

BF

BFα

α

+

−=



Central Laser Facility Annual Report n 2005/2006142

5Results and DiscussionWhen a positive muon is implanted in a molecule ormaterial it is called a muoniated species. Our initialexperiments explored the possibility of studying theelectronic excitation of such a muoniated centre in amolecule formed by addition of muons to rose bengal(4,5,6,7-tetrachloro-2’,4’,5’,7’-tetraiodofluorescein,C20H2Cl4I4Na2O5, - RB), a halogenated xanthene dye(Figure 1), which has been used as a photosensitizer inphotodynamic therapy (PDT) [2]. The photochemistry ofRB has been extensively studied and several photophysicalparameters associated with its triplet and singlet stateshave been determined. It is known that in the presence ofH+ there is a red shift in the RB absorption spectrum [3].The same effect has also been observed for otherhalogenated and non-halogenated dyes, which show a redshift of ~50 nm in the protonated form [4]. This could beconsidered similar to the case where µ+ is added to RB,

since the chemistry of µ+ containing molecules is similarto the chemistry of H+ containing molecules, except forpossible quantum effects such as zero point vibrationaland tunneling effects. During this experiment (Figure 2)the muon signal amplitude (asymmetry) in powdered rosebengal is monitored with and without 532 nm laserexcitation (20 mJ at the sample in 20 × 20 mm beam) atzero magnetic field (Figure 3). RB has a strong absorptionclose to 532 nm due to a strong π → π* transition. Thesubsequent analysis shows a statistically significant effect(Figures 3 and 4), with very clear differences between thetwo spectra shown in Figure 3. When a magnetic field isapplied along the direction of spin of muons or in ZeroField (ZF) environments, in the absence of a localmagnetic field, there is no precession since the appliedfield (if any) and muon spin polarisation are co-linear.Under these conditions, each muon is in a stationary state.If a local magnetic field is present the time evolution ofthe muon spin can be measured in ZF. The ZF laser-µSRdata such as that presented here can make a uniquecontribution to the field of transient excited statechemistry; similar measurements such as ESR aregenerally performed in large applied magnetic andresonating radio frequency fields. With this spectroscopictechnique we should be able to tune the laser frequency toinduce an electronic transition or a photochemicalreaction that would lead to a muon-electron hyperfineinteraction. In this way, we will be able to characterize

Figure 1. The optimized structure of rose bengal at AM1level of the theory.

Figure 2. The schematic of the setup for the experiments onrose bengal powder. The laser and muon window is a thin(0.1 mm) quartz cover slip.

Figure 3. The asymmetry in powdered RB with and without532 nm laser irradiation (0.3 µs delayed relative to the muonpulse). The curves represent a Lorentzian fit (for the data inthe absence of laser excitation) and Lorentzian withoscillatory component (for the data in the presence of laserexcitation). The sizes of the error bars are smaller in thelaser off experiments due to the longer laser off periodcompared to laser on (hence the higher statistics of the laseroff data).


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both the ground state, by following the effect of the laseras a function of frequency, and the excited states, throughtheir hyperfine interactions. Moreover we can characterizethe transient species in a time-resolved manner. Forexample, the asymmetry of RB without and with laserexcitation at selected time delays is shown in Figure 5. Theobservations in Figures 3-5 suggest that laser irradiationwill generate local magnetic fields which the muon sensesand in which it oscillates after the onset of laserirradiation. The unpaired electrons of free radicals couldgenerate such local magnetic fields through anisotropichyperfine interactions. However, typical µSR free radicalsignals decay significantly in a fraction of a microsecond[1]. The fact that we observed the laser induced oscillations

even after 1 µs suggests that the oscillations of the muonspin polarization are due to the excitation of muoniateddiamagnetic molecules to the singlet state, followed byintersystem crossing to the triplet state. Muoniateddiamagnetic molecules are stable over the muon lifetime,and therefore excitation of these muoniated centres ispossible during their lifetime. The laser-inducedoscillations are therefore due to muon-electron dipolarhyperfine interactions in the triplet state.

In the results reported above laser radiation is used toexcite a transient, produced by muon irradiation, itsresponse to laser excitation is then followed with µSR viaits hyperfine interactions in the excited state. With thisproof of principal work we have set out to develop atechnique to study the chemistry of the transientsinvolved in PDT. This technique also has the potential tobe used for the investigation of laser-materialinteractions.

In the following we report another novel experiment,pump-probe study, in which the system is excited by laserradiation and the chemical dynamics are probed usingµSR. The aim of this second study was to investigate thechemical dynamics of the reaction between Mu andmolecules in excited states, in particular electronic excitedstates, which are just one of the many transientintermediates that play an important role in PDT.

The asymmetry parameter, A(t), in a transverse fieldexperiment includes contributions from paramagnetic Mu,as well as free radical and diamagnetic molecules [1,5]:

(3)

))(1()(/

0 tAeNtNt

+=− μτ

)()(

)()()(

tNtN

tNtNtA

BF

BFα

α

+

−=



Figure 4. Difference between laser on and off asymmetry atzero magnetic field in powdered RB with 532 nm laserirradiation (0.3 µs delayed relative to the muon pulse).

Figure 5. The asymmetry in powdered rose bengal with and without 532 nm laser irradiation (20 mJ/pulse) at differentpositive time delays relative to the muon pulse.


Central Laser Facility Annual Report n 2005/2006144

5where t is time, Ai the asymmetry of the fraction i in itsgiven environment, λi the relaxation rate of the muon spinin that environment, wi the corresponding precessionfrequency, and ϕi the initial phase for this fraction. Theparameters of interest, Ai, λi, and wi are extracted from fitsof equation 3 to experimental data, and provideinformation on radiolysis reactions, kinetics, and hyperfineinteractions (in Mu or muoniated radicals). At ISIS, theMu amplitudes are most easily found in a weak field ofaround 2 G. In these low fields the two muonium “triplet”frequencies are very close and unresolved, so theasymmetry is fitted with a single oscillating term.

Experiments were conducted using aqueous solutions ofRB over the concentration range 10–5 M to 5 × 10–5 M at298 K and 1 bar external pressure, and in the presence of a2 G magnetic field transverse to the muon spinpolarization. Timing was arranged so that the beginning ofthe muon pulse was concurrent with laser excitation.Figure 6 presents an example of the observed asymmetryin a 5 × 10-5 M aqueous solution of RB with and without532 nm laser excitation (10 mJ at the sample in 2 × 20 mmbeam). The ratio of photons (per pulse) to RB moleculesin the path of laser beam is almost 1/1. The curves are fitsof equation 3 to the experimental data. The relaxation

rates (λi in equation 3) corresponding to the reaction ofMu with RB were always increased after laser irradiation,as can be seen in the example from Figure 6. Thus, the rateconstant for Mu reaction (k2) with RB in its lowestelectronic excited state (triplet state) is larger than thereaction with RB in the ground state (k1). Measurementsat different concentrations suggest k2/k1 = 2.0 +/– 0.3. Inthe future, by comparing Mu reaction rates with those ofatomic hydrogen or deuterium, we can apply laser-muonspin spectroscopy to the study of KIEs as a function ofexcitation energy for a variety of reactions. All studies ofKIEs for Mu reactions have so far involved only moleculesin their ground states. Therefore, when it comes to reactiondynamics, the ability to perform studies under laserexcitation, as exemplified by our results presented inFigure 6, is important in initiating studies of the mostsensitive KIEs (due to large mass ratios H/Mu and D/Mu)in the excited states.

In another pump-probe study we used rose bengal as aphotosensitizer (its method of action in PDT) totransform ground state (triplet) O2 to singlet O2 andcompared the rate of reaction of Mu with triplet O2 ascompared to singlet O2. Figure 7 presents an example ofthe observed asymmetry in a 10-5 M aqueous solution ofO2 and 10-5 M RB with and without (Figure 7b) 532 nmlaser excitation. The curves are fits of equation 3 to theexperimental data. Again there are very clear differencesbetween the two spectra.

Figure 6. (a) The asymmetry in 5 × 10–5 M RB aqueoussolution at 2 G; relaxing signal is due to Mu addition to thearomatic ring of RB in the ground state (in the absence oflaser irradiation). (b) The asymmetry in 5 × 10–5 M RBaqueous solution at 2 G when irradiated with 532 nm laserradiation with the beginning of the muon pulse concurrentwith laser excitation; the curves are fits of equation 3 to thedata. The sizes of error bars are smaller in laser offexperiments due to longer laser off period as compared tolaser on (hence the higher statistics of the laser off data).

Figure 7. (a) The asymmetry in 10–5 M O2 and RB aqueoussolution at 2 G; relaxing signal is due to Mu electron spinexchange with oxygen electron (in the absence of laserirradiation). (b) The asymmetry in 10–5 M O2 and RBaqueous solution at 2 G when irradiated with 532 nm laserradiation with the beginning of the muon pulse concurrentwith the laser excitation; the curves are fits of equation 3 tothe data. The reaction is addition of Mu to singlet O2.


5ConclusionIn summary we have developed two novel techniques,laser-muon spin spectroscopy and laser pump-muonmagnetic resonance probe, which combine the facilitiesavailable within the Central Laser Facility and the µSRtechniques available within the ISIS facility at RAL. Thesetechniques, which, for the first time, extend muonchemistry into the realm of excited states, have a widevariety of potential applications in the fields of radiationchemistry, chemical dynamics and free radical chemistry.The techniques will allow the study of the dynamics ofdifferent intermediates generated during muonthermalization. This would be a unique contribution to thefield of non-homogeneous chemical dynamics andreactions under far from equilibrium conditions.Eventually, these studies would allow us to fullycharacterize the end of track chemistry involved inradiolysis processes.

Our next generation of experiments will study Mureactivity, similar to the studies of the reactions of Muwith O2. One experiment of this nature would be to pumpa molecule to a vibrational excited state with a tuneablelaser, for example H2O (v = 4)*. We would then comparethe Mu relaxation rate of the Mu + H2O (v =4)* reactionwith that of ground state H2O. The latter reaction isextremely slow even at temperatures as high as 400°C andpressures as high as 400 bar [6]. Since Mu + H2O is quiteendoergic, large zero point energy shifts are expected at thetransition state. These will be offset by vibrationalexcitation , yielding orders of magnitude enhancement inrate, a well-known effect in kinetics studies. Such studies ofKIEs as a function of excitation energy have never beenperformed before, since all Mu reactions that have beenstudied so far involve molecules in their ground states. Ourresults presented in Figures 6 and 7 are important inopening the door to studies of the most sensitive KIEs inthe excited states. Also interesting, but very difficult withpresent muon intensities, is the use of laser-inducedfluorescence spectroscopy to measure final statedistributions where recent developments in pulsed lasersystems in the NIR region might allow for the observationof states such as MuO* (formed, for example, in the Mu +H2O* reaction in the gas phase).

In the realm of free radical chemistry in excited states, ourstudies of triplet states could be extended to studies ofmuoniated free radicals (both organic and inorganic) indifferent high spin excited states. A combination of thesestudies with reaction dynamics studies in triplet states aswell as mechanisms of reactions under laser excitationswill be valuable for further PDT developments. Anotherapplication would be studies of laser-inducedmagnetization, which is similar to laser-induced ZF-µSRoscillations of the present experiments. However, in thiscase interactions would result from changes in magneticinteractions between molecules rather than localexcitations. Also, from the fundamental perspective, ourcurrent studies open the door for future studies ofhyperfine interactions in selected excited states, inparticular selected vibrational and electronic excited states.

References1. Roduner, E., Appl. Magn. Reson., 13, 1, 19972. Conlon, K.; Rosenquist, T.; Berrios, M., Mol. Bio. Cell,

13, 35A, 20023. Liu, S. P.; Feng, P.; Microchim. Acta, 140, 189, 20024. Gorbenko, G. P.; Mchedlov-Petrossyan, N. O.;

Chernaya, T. A., J. Chem. Soc. Faraday T., 94, 2117,1998

5. Ghandi, K.; Bridges, M. D.; Arseneau, D. J. Fleming,D. G., J. Phys. Chem. A; 108, 11613, 2004

6. Ghandi, K.; Brodovitch, J.-C.; Addison-Jones, B.;Schüth, J.; Percival, P. W., PCCP, 4, 586, 2002


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