researchaccomplishmentsvandana_30dec

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Research Accomplishments

Summary of my PhD work:

My PhD training in the field of atomic and molecular collisions physics involved

electronic excitation and ionization in atoms and molecules. In particular, my thesis

focused on fragmentation dynamics of highly charged triatomic and polyatomicmolecular ions created by high energy electron impact on neutrals (CO2, CCl4, SF6,

alcohols etc). In the following subsections I will briefly summarize my PhD work,

starting with the significance of topic, the instruments developed for the study and theexciting new results from the work.

Significance of the study:

Molecular ions are important to study as they have an astrophysical relevance. They are

found in places of extreme conditions of temperature or plasma activity. They exist in the

sun, around comets, and throughout the galaxy in stars, supernova remnants, in theinterstellar medium and in more distant external galaxies. They provide a diagnostic of

temperature and plasma density in the region in which they exist.

Molecular ions are usually unstable, although some may be metastable. Very little is

known about these highly charged intermediate molecular ions because of their transient

nature, which obscures investigation by conventional spectroscopic techniques. However,

the properties of these transient species are carried by the momenta of the fragmentsarising from them. Imaging the momenta of the fragments is thus a proven tool for

studying these molecular ions.

Instrumentation: The Momentum Imaging Spectrometer

To carry out these experiments, I designed and built an imaging momentum spectrometerin-house for the first time in India. It has the capability to record several fragment ions

arising from a single break-up event of a molecular ion [1]. Recording multiple ions is

mandatory for the study of the complete fragmentation dynamics. The spectrometer has amomentum resolution of 12.4 a.u. (1 a.u. is the root-mean-square momentum of an

electron in the ground state of the H atom and is equal to 2.04 X 10-24

kg-m/s). This

resolution, limited by instrumental uncertainties and thermal motion of the molecular

target, is good enough to resolve the momenta of the fragment ions as they have momentawhich are at least two orders of magnitude higher than the mean momenta of the neutral

target or the precursor molecular ion prior to dissociation.

An event-by-event reconstruction of the momentum space spanned by the fragments

makes it possible to study various aspects of dissociation, such as identification of

channels, lifetime of metastable species, kinetic energy release, angular distribution offragments and preferential bond breaking. I have carried out some quantum chemical

computations in conjunction with the experiments, to support the experimental results[2].


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Results: Site specific nature of fragmentation

It was shown experimentally that the symmetry of a molecular ion is often different from

that of the ground state of the neutral. For the first time it was shown that for symmetric

polyatomics (CCl4 and SF6) the change was very substantial and to involve migration ofatoms. In asymmetric polyatomic molecules which have a few identical atoms attached to

different skeletal atoms, such as ethanol, association of atoms during dissociationionization was not only shown to occur, but also its site-specific nature was

demonstrated. By a site-specific analysis of the fragmentation patterns, it was shown, for

the first time, that bond strength alone is not a sufficient parameter for determining the

propensity to dissociation [3,4,5].

Research activity following the PhD:

My work as a Post Doctoral Fellow at Joint Institute of Laboratory Astrophysics (JILA),

University of Colorado at Boulder USA and Max Planck Institute for Nuclear Physics(MPI-K), Heidelberg, Germany, concerns experiments regarding interaction of ultra-shortlaser pulses with atoms and molecules. In the following paragraphs, I will summarize the

work carried out at JILA and MPI-K, to which I have contributed significantly.

Autoionization in small molecules:

One of my post doctoral projects at JILA, University of Colorado at Boulder, USA,concerned femtosecond molecular dynamics driven by extreme ultra violet (EUV)

radiation. 15-femtoseconds (fs) EUV radiation was created by up-converting ultrashort

(30 fs), high-intensity, infrared (IR) laser pulses through the mechanism of high-

harmonic generation. It was then used to initiate dynamics in molecules through valenceionization, inner-valence ionization and shake-up processes. The dynamics were probed

by time-delayed IR pulses. An ion and electron coincident momentum imaging detector

was used to obtain the three-dimensional momentum spectra of all the fragments. Thiscombination of high-harmonic generation and momentum imaging technologies has

allowed us to observe for the first time the dissociation dynamics of an autoionizing state

of O2 [6]. We found that autoionization after EUV photoionization of molecular oxygenfollows a complex multistep process. By interrupting the autoionization process with the

IR laser pulse, we established that autoionization cannot occur until the internuclear

separation of the fragments is greater than approximately 30 angstroms. As the ion andexcited neutral atom separated, we directly observed the transformation of electronically

bound states of the molecular ion into Feshbach resonances of the neutral oxygen atomthat are characterized by both positive and negative binding energies. An exciting

discovery was the existence of states with negative binding energies, which have not beenpredicted or observed in neutral atoms so far.


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Controlling the dynamics of dissociation with few-cycle laser pulses:

Currently I am working on the dissociation of H2 molecules by few-cycle (6 fs, 760 nm)

linearly-polarized, intense (0.44 PW/cm2) IR laser pulses with carrier-envelope-phase

(CEP) stabilization and control. CEP is a parameter by which the actual waveform of a

few-cycle pulse, under the pulse envelope, can be controlled. We observed anasymmetric proton emission in the laboratory frame, which depends on the CEP and

molecular orientation. The reason for the observed asymmetry is the coupling betweenthe 1sg and the 2pu molecular states of the H2

+ion. These states transfer population

between them, and, depending on the CEP (i.e., the waveform) of the laser pulse, the

population of the localized states can be controlled, leading to the observed asymmetry.

Although the laser field is rather weak in the coupling region, the appearance of adissociation asymmetry proves that the laser-induced coupling is strong enough to

effectively control the symmetry of the dissociation process. The salient features of the

experimental results are reproduced by wave packet propagation calculations [7].

Future Goals:

Ultrafast molecular dynamics studied with momentum spectroscopy: an overview

The visualization of ultrafast changes in the electronic structure of atoms and molecules

as a function of time is at the frontier of research in atomic and molecular physics.

Studies of such ultrafast dynamics require the acquisition of snapshots of atomic or

molecular observables at different times. Exposure times have to be shorter than thecharacteristic timescale of the dynamic process under study and generally shorter

exposure times imply better time resolution. Currently, many labs all over the world

operate intense ultrashort lasers with few fs pulse lengths in the visible to the IR and evenin the THz domain. However, photon energies in these wavelength ranges are insufficient

to access inner valence states. With high-harmonic up conversion, sufficiently high-

energy photons can be generated to access these states. The main drawback of HHG isthe fact that pulse intensities are orders of magnitude lower than the intensities at the

fundamental wavelength, imposing severe limitations on observation of low cross section

processes. With the advent of EUV and soft X-ray FEL sources, intense pulses with

unprecedented peak brilliance and femtosecond to attosecond pulse duration in thisenergy range are becoming available presently. KVI is aiming to construct a soft-X-ray

free-electron-laser system in the near future which provides an ideal opportunity to

perform time-resolved studies of molecular dynamics. In continuation to the work I

carried out as a Post Doctoral Fellow, in the future I plan to set-up a reaction microscopeto study the X-ray induced time-resolved dissociation dynamics of molecules. As

mentioned above, such coincidence measurements of ion and electron momenta havegiven us unprecedented information about fragmenting molecules. The members of MPI-

K Heidelberg group were pioneers in establishing the technique of Reaction Microscopy1,

and have made tremendous progress in simultaneous detection of ion and electron

1 Also known as Cold Target Recoil Ion Momentum Spectroscopy (COLTRIMS)


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momenta in laser-matter interaction and particularly with novel light sources such as the

FEL.

FEL pulse characterization:

In the self amplified stimulated emission (SASE) FEL process, lasing starts from anindividual electron bunch, leading to a shot-by-shot variation of the laser pulses

(intensity, temporal and spectral variation) and even for the case of a seeded FEL asplanned at KVI (ZFEL), such characterization and control of these variations is crucial.

Precise characterization of FEL pulses is the basis for time-resolved studies. Furthermore,

these properties have to be known in order to fully characterize FEL operation itself. At

the FEL source at Hamburg (FLASH) [8], intensity autocorrelation is the technique usedto measure the average pulse duration. Intensity autocorrelation is a well-established and

direct method to study pulse durations based on the nonlinearity of ionization processes

in atoms and molecules. Since ZFEL at KVI will be a new facility, precise pulsecharacterization will be an absolute requirement. An ideal approach for pulse

autocorrelation determination is the interaction of two time-delayed FEL pulses with agaseous target in the center of a Reaction Microscope. Multiphoton ionization, occuringin all atoms or molecules in high fluence photon fields is characterized by an ionization

rate that depends nonlinearly on the intensity of the incident field. Measuring

photoionization yields as a function of time delay will thus directly yield the pulseduration of the FEL. The characterization of the FEL pulses is not only a requirement for

understanding the dynamics of atoms or molecules in FEL-based experiments, e.g. by

comparison with ab initio calculations, but will be of key importance for the operation of

ZFEL itself as well for a multitude of other experiments.

Molecular dissociation dynamics/Interamolecular atomic migration:

Besides the experiments aiming at pulse characterization, I plan to pursue a series of

experiments that seek to follow complex ultrafast x-ray driven dynamics in polyatomicmolecules. Being a momentum imaging technique, the Reaction Microscope apparatus is

ideally suited for following dynamically the changing molecular structure. On a technical

front, such studies are often challenging, for e.g. in acquiring and storing enormousamounts of coincidence data and the required fs time-correlation of ZFEL and Reaction

Microscope.

As an example, a particularly interesting dynamic process in molecules is molecular

elimination following ionization. An example for this process is CCl4 dissociating into

CCl2 and Cl2 molecules: after electron impact ionization [4], a Cl atom splits off the

parent CCl4 molecule and creates a new bond with a bound Cl atom [9] before themolecular elimination of Cl2 takes place. There are no time resolved measurements

available to date to convincingly demonstrate this channel. It would be very interesting to

do a time-resolved measurement of this process that shows that this reaction indeedproceeds via an intermediate state with a changed molecular structure before Cl2 is

eliminated. The capability of the reaction microscope to detect multiple coincidence ions

and electrons is ideal for studying such polyatomic molecules. Various pump-probe


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(EUV pump EUV probe, EUV pump IR probe, IR pump EUV probe) experiments

can be explored to study photochemical reactions that may result in an intra-molecularelimination. There are various such reactions which could be of interest to the atomic

and molecular physics and chemistry community e.g. ultrafast isomerization in acytelene

[10, 11, 12], hydrogen migration in methanol [13] etc.

Reaction Dynamics (non-unimolecular reactions):

Looking further into the future, a broad unexplored area is molecular reaction dynamics

of different molecules which have atmospheric and biological relevance in the presence

of a foreign body such as an aerosol surfaces (volcano ash, dust particles). Until now, no

molecular reaction dynamics study has been able to completely characterize suchreactions. For instance, until now, there is little understanding of the chemistry of inter-

planetary atmosphere, particularly in the presence of aerosols and EUV [14].

Conclusion:

In conclusion, I plan to implement a technique to fully characterize ZFEL pulses, basedon a ZFEL beam line equipped with a Reaction Microscope. This apparatus will also

allow us to study X-ray/EUV induced molecular dissociation.

To start with, I propose time-resolved pump probe experiments on moleculareliminations. These provide a means for finding out timescales of formation and decay of

intermediate states which are experimentally and theoretically challenging. By combining

advanced detection techniques of reaction microscope, which images the momentum of

charged particles, with novel photon sources such as ZFEL, we will be able to followdynamically changing molecular structure in unprecedented detail on ultrafast timescales.

References:

1. Sharma and Bapat,European Physical Journal D, 37, 223 (2006)2. Sharma et al.,J. Phys Chem., 101, 10205 (2007).3. Sharma and Bapat,J. Chem. Phys., 125, 044305 (2006)4. Sharma and Bapat,J. Phys. B, 40, 13 (2007).5. Sharma and Bapat, Phys. Rev. A, 75, 040503(R) (2007)6. Sandhu et al., Science, 322, 1081 (2008)7. Kremer et al., Phys. Rev. Lett., 103, 213003 (2009)8. Jiang et al., Phys. Rev. A, 82, 041403 (2010)9. Garca de la Vega et al.,J. of Phys. Chem., 99, 12135 (1995)10.Landers et al., Phys. Rev. Lett., 87, 013001 (2001)11.Osipov et al., Phys. Rev. Lett., 90, 233002 (2003)12.Hishikawa et al., Phys. Rev. Lett., 99, 258302 (2007)13.De et al., Phys. Rev. Lett., 97, 213201 (2006)14.Pandey and Bhattacharya,J. Chem. Phys. 124, 234301 (2006).

researchaccomplishmentsvandana_30dec

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