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1996 |
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TIME-RESOLVED MOLECULAR DYNAMICS
VOLKER ENGEL
Institut für Physikalische Chemie, Universität Würzburg,
Marcusstr. 9-11
97070 Würzburg, Germany.
Time-resolved experiments have been performed on a diversity of
molecular systems. Applications of spectroscopic techniques which
work in the time-domain range from the detection of simple
vibrational motion of a diatomic molecule to the direct determination
of relaxation times in polyatomic molecules in a liquid environment
or the recording of isomerization processes in biomolecules.
Although very different molecular systems have been
investigated with various experimental techniques,
the principles of the experiments are more or less
the same.
Below a brief description of the basic ideas is given with the
emphasis on gas phase molecules under collision-free conditions as
usually provided in a molecular beam.
in Atomic, Molecular and Optical Physics
Handbook, edited by G.W.F.Drake , AIP Press,Woodbury, 1996.
MAPPING OF MOLECULAR WAVE-PACKET DYNAMICS BY
TIME-RESOLVED FEMTOSECOND PHOTOELECTRON SPECTROSCOPY
MICHAEL BRAUN AND CHRISTOPH MEIER
Fakultät für Physik, Universität Freiburg, Hermann-Herder-Str. 3
79104 Freiburg, Germany
VOLKER ENGEL
Institut für Physikalische Chemie, Universität Würzburg, Marcusstr. 9-11
97070 Würzburg, Germany
We calculate the energy distributions of photoelectrons which are produced
by femtosecond pump/probe ionization of small molecules. Several examples
show, that these spectra reveal details about the quantum dynamic of prepared
wave packets.
in Femtochemistry-Ultrafast Chemical and Physical
Processes in Molecular Systems, edited by M. Chergui , World Scientific,
Singapore:p.78, 1996.
MOLECULAR PHOTOIONIZATION WITH A SINGLE INTENSE FEMTOSECOND LASERPULSE:
A TEMPORAL DIFFRACTION EXPERIMENT
CHRISTOPH MEIER
Fakultät für Physik, Universität Freiburg, Hermann-Herder-Str. 3
79104 Freiburg, Germany
VOLKER ENGEL
Institut für Physikalische Chemie, Universität Würzburg, Marcusstr. 9-11
97070 Würzburg, Germany
Kinetic energy distributions of electrons obtained from
photoionization of Na2 with strong femtosecond laserpulses of different wavelengths
are calculated.
Strong pulses induce Rabi-oscillations between
electronic states of the molecule. The temporal
variation of the population in the state which is resonantly
coupled to the ionization continuum is transformed into interference
patterns in the electron spectra. The appearance of the structures change
if two or three electronic molecular states are involved in the
ionization process. The analogy to a diffraction experiments is discussed.
in Femtochemistry-Ultrafast Chemical and Physical Processes in Molecular
Systems, edited by M. Chergui , World Scientific, Singapore:p.667, 1996.
VIEWING THE REACTION PATH WITH THE HELP OF TIME-RESOLVED FEMTOSECOND
SPECTROSCOPY
CHRISTOPH MEIER
Fakultät für Physik, Albert-Ludwigs-Universität
79104 Freiburg i. Br., Germany
VOLKER ENGEL
Inst. für Physikalische Chemie
Universität Würzburg
97070 Würzburg, Germany
The spectroscopy of the transition state , which separates products from
educts in a chemical reaction has been the subject of numerous
studies. A beautiful idea, which by now has been applied to a
great variety of molecular systems is to use electron detachment spectroscopy
to directly resolve resonance features which are the fingerprints of
quasi-bound states of the molecule. The latter
are localized in the transition-state
region. Nevertheless this kind of spectroscopy does not tell us about
the history of the system when it evolves from this intermediate region
towards the product channels. In this chapter we want to line out, from
a theoretical point of view, how time-resolved spectroscopy can
be used to monitor the evolution of a system which is prepared in
the transition state, until it ends up in several product channels.
This idea has been promoted by Zewail and coworkers over the last years
and exciting experiments were performed. It is not the purpose of this
contribution to review those experiments.
Rather, we will describe the principles of time-resolved
spectroscopy using quantum mechanical and classical
theory. In particular it will be seen that the motion along the
reaction path can be directly taken from an experimental signal.
The article is organized as follows: in Sec. 2 we outline the scheme of
a pump/probe experiment which is performed with two ultrashort laser pulses.
Then the excitation mechanism which prepares the system in a coherent superposition of
eigenstates in the transition-state region of the respective potential
surface is described (Sec. 3). This localized wave packet evolves along the
reaction path, as described in Sec. 4. In Sec. 5 it is shown how the
wave-packet motion can be probed. A short summary concludes this chapter.
in The reaction path: concepts and methods, edited by D. Heidrich ,
Kluewer, Dordrecht, 1996.
NANOSECOND WAVE-PACKET PROPAGATION WITH THE SPLIT-OPERATOR-TECHNIQUE
MICHAEL BRAUN, CHRISTOPH MEIER
Fakultät für Physik, Albert-Ludwigs-Universität
79104 Freiburg im Breisgau, Germany
VOLKER ENGEL
Institut für Physikalische Chemie
Universität Würzburg
97070 Würzburg, Germany
We investigate the grid propagation of quantum mechanical wave packets for
propagation times of several nanoseconds using the
"Split-Operator-Technique".
The vibrational motion of a diatomic molecule is chosen as a
numerical example. For the systems under investigation the propagation time
is several thousand times larger than the vibrational period.
Physical properties calculated as a function
of time show only small numerical errors. Even for
timesteps which are chosen too big to keep the error in the
phase of the wavefunction small, surprisingly good results can be obtained
in the average.
Comput. Phys. Commun., 93,152 (1996).
WAVE-PACKET INTERFERENCE AND STABILIZATION OF A PREDISSOCIATING MOLECULE
H. DIETZ AND V. ENGEL
Institut für Physikalische Chemie
Universität Würzburg
Marcusstr. 9 - 11
97070 Würzburg
GERMANY
We investigate the wave-packet dynamics in a diatomic molecule which decays
via electronic predissociation. It is shown that the amplitudes corresponding to
different pathways in a model curve-crossing system can interfere constructively
so that the dissociation channel becomes closed and the molecule is stabilized.
The possibility to observe this effect is discussed for femtosecond excitation of the
NaI molecule.
Chem. Phys. Lett., 255, 258 (1996).
THE REFLECTION OF PREDISSOCIATION DYNAMICS IN PUMP/PROBE PHOTOELECTRON
DISTRIBUTIONS
MICHAEL BRAUN, CHRISTOPH MEIER
Fakultät für Physik
Albert-Ludwigs-Universität
79104 Freiburg i. Br., Germany
VOLKER ENGEL
Institut für Physikalische Chemie
Universität Würzburg
97070 Würzburg, Germany
We present simulations of pump/probe photoionization experiments on a diatomic molecule
with two non-adiabatically coupled electronic states. The
NaI molecule is used as an example. The nuclear wave packet dynamics in two coupled
electronic states is mapped into the kinetic energy distribution of
photoelectrons which are recorded at different delay times between the pump
pulse, preparing the initial wave packet and the ionizing probe pulse. In this
way details of the indirect fragmentation process can be observed in real time.
J.Chem.Phys., 105, 530 (1996).
SHORT-TIME WAVE-PACKET DYNAMICS AND THE REFLECTION PRINCIPLE OF
CONTINUUM RESONANCE RAMAN SCATTERING
M. EICHELSDÖRFER AND V. ENGEL
Institut für Physikalische Chemie
Universität Würzburg
Marcusstr. 9 - 11
97070 Würzburg
GERMANY
We use the time-dependent approach to calculate cross sections for
continuum resonance Raman scattering to derive the Raman reflection principle.
The essential step in the derivation is to use a short-time
approximation to the quantum mechanical propagators.
Raman scattering from a diatomic molecule
serves as an example to compare our approximate treatment with
numerically exact results and a comparable formula derived by Lee
(Chem. Phys. Letters 203, 93 (1993)).
Chem. Phys. Lett., 263, 640 (1996).
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1997 |
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STIMULATED EMISSION PROCESSES AND STRONG FIELD EFFECTS IN
ULTRASHORT PULSE EXCITATION OF A PREDISSOCIATVE MOLECULE
H. DIETZ, A. MATERNY, AND V. ENGEL
Institut für Physikalische Chemie
Universität Würzburg
Marcusstr. 9 - 11
97070 Würzburg
GERMANY
We study the interaction of a diatomic molecule with two ultrashort and strong
laser pulses. The first pulse prepares a wave packet in a dissociative
electronic state which is coupled non-adiabatically to the bound electronic
ground state. The conditions under which it is possible to stimulate
parts of the wave packet back down to the ground state, a process which competes
with excitation to a higher electronic state, are investigated.
The results show to which extent it is possible to selectively transfer
population between bound and dissociative states and thus influence the
yield of a simple chemical reaction.
Chem. Phys., 217, 249 (1997).
PUMP/PROBE DIRECT PHOTOIONIZATION FROM THERMALLY HOT SAMPLES:
THE C2 MOLECULE
M. BRAUN AND V. ENGEL
Institut für Physikalische Chemie
Universität Würzburg
Marcusstr. 9 - 11
We investigate the femtosecond photoionization of Cs2 within a model
including four electronic states. A thermally broad
initial state distribution influences the time-dependence of the transient
ion signals crucially. We show that even for the present case of simple one-dimensional
motion the interpretation of ionization signals requires special care under certain
experimental conditions.
The results are compared to recent experimental data and a modified ionic state
potential is constructed.
Z. Phys. D-Atoms, Molecules and Cluster, 39, 301 (1997).
INVERSION OF POTENTIAL ENERGY CURVES FROM CONTINUUM RESONANCE
RAMAN SCATTERING DATA WITH THE HELP OF A REFLECTION PRINCIPLE
M. EICHELSDÖRFER AND V. ENGEL
Institut für Physikalische Chemie
Universität Würzburg
Am Hubland
97074 Würzburg
GERMANY
We present an inversion method for diatomic potential curves of electronically
excited states which are the intermediate states for continuum resonance Raman
scattering processes. Assuming a parametric form of the potential, the
parameters are directly given as a function of certain frequencies which
can be taken from experimentally determined Raman profiles. The origin of
the procedure is a reflection principle which states that the nodal
structure of vibrational wave functions serving as initial or final
states in the two-photon
process is aparent in the Raman cross section recorded as a function
of excitation frequency. The Cl2 molecule is used as an illustrative example.
J. of Raman Spectrosc., 28, 427 (1997).
VIBRATIONAL DYNAMICS AND ENERGY FLOW
IN A MULTI-MODE ORGANOMETALLIC COMPOUND
T. MARKERT*, W. MALISCH+, AND V. ENGEL*
*Institut für Physikalische Chemie
Universität Würzburg
Am Hubland
97074 Würzburg
GERMANY
+Institut für Anorganische Chemie
Universität Würzburg
Am Hubland
97074 Würzburg
GERMANY
We present classical and quantum mechanical calculations on the vibrational
dynamics of an organometallic fragment. Using data from Raman measurements
a simple potential energy surface is constructed for the electronic ground state.
The excitation with strong infrared femtosecond
laserpulses is simulated. The transfer of vibrational energy between
different modes which takes place after the field-molecule interaction is investigated.
It is shown that energy can be exchanged between
certain bonds while others are almost completely decoupled.
Chem. Phys. Lett., 270, 222 (1997).
THE CALCULATION OF TIME-RESOLVED NENEPO-SPECTRA WITH AN APPLICATION TO IRON-CARBONYL
O. RUBNER, C. MEIER, AND V. ENGEL
Institut für Physikalische Chemie
Universität Würzburg
Am Hubland
97074 Würzburg
GERMANY
We investigate "negative-ion-to-neutral-to-positive-ion" (NENEPO) pump-probe
spectroscopy as proposed by Wöste and co-workers
(Wolf et. al. Phys.Rev. Lett. 74, 4177 (1995))
where the dynamics of wave packets in a neutral molecule produced by photodetachment
from the negative ion is
probed by time-delayed photoionization and detection of positive molecular ions or
photo-electrons. Approximations are introduced for an effective
numerical treatment of the double-ionization process.
The organometallic compound FeCO is used as an example.
J. Chem. Phys., 107, 7749 (1997).
THE PERTURBATION OF COHERENT WAVE-PACKET DYNAMICS BY ATOM/MOLECULE
COLLISIONS: THE NaI + Ar SYSTEM.
H. DIETZ, G. KNOPP, A. MATERNY, AND V. ENGEL
Institut für Physikalische Chemie
Universität Würzburg
Am Hubland
97074 Würzburg
GERMANY
The results of recent femtosecond pump/probe experiments on the NaI molecule
interacting with Ar atoms under different pressure conditions
(Knopp et al., J. Phys. Chem., in press)
are interpreted using classical and quantum mechanical
calculations. It is shown how the periodic quasibound NaI wave-packet
dynamics initiated by ultrashort pulse excitation is modified
through the collision process and how the pump/probe fluorescence
signal changes accordingly. Furthermore it is demonstrated how the
energy transfer in the collision processes yields internally cooled NaI
molecules which are more stable with respect to electronic predissociation.
Chem. Phys. Lett. , 275, 519 (1997).
SEMICLASSICAL PHOTODISSOCIATION CROSS SECTION FOR H2O
B. HÜPPER, B. ECKHARDT
Fachbereich Physik
Universität Marburg
35032 Marburg
GERMANY
V. ENGEL
Institut für Physikalische Chemie
Universität Würzburg
Am Hubland
97074 Würzburg
GERMANY
We investigate semiclassically the photodissociation of water in the first
absorption band. Experimental absorption spectra show weak undulatory
structures, which are related to the vibrational symmetric stretch motion on
the excited Ã-state potential-energy surface (PES). using the cycle
expansion resummation we demonstrate that there are only small corrections
due to other fundamental periodic orbits and negligible contributions from
longer orbits. The error in the positions of the semiclassical resonances is
of the same order or less as the variations when using different Born-
Oppenheimer potential surfaces. For this light molecule the background term
is strongly influenced by quantum effects. We discuss estimates for the
importance of quantum corrections and a way to incorporate them.
J. Phys B: At. Mol. Phys. , 30, 3191 (1997).
FEMTOSECOND TIME-RESOLVED TWO-PHOTON IONIZATION SPECTROSCOPY OF
K 2
H. SCHWOERER, R. PAUSCH, M. HEID, V. ENGEL, AND W. KIEFER
Institut für Physikalische Chemie
Universität Würzburg
Am Hubland
97074 Würzburg
GERMANY
We investigated the coherent motion of vibrational wave packets in the
|B> 1
u state of the potassium dimer applying
two color
pump/probe spectroscopy with a sub 100 fs time resolution. Special
interest was paid to the ionization probe step which was analyzed carefully
by varying the probe energy over a wide range. Time-dependent quantum
calculation explain the experimental outcomes by introducing a nonconstant
transition dipole moment between the |B> and the ionic state
|X +> and by taking into account the excitation of long lived
autoionizing Rydberg states.
J. Chem. Phys. , 107, 9749 (1997).
VIBRATIONAL REVIVALS AND THE CONTROL OF PHOTOCHEMICAL REACTIONS
S. MEYER AND V. ENGEL
Institut für Physikalische Chemie
Universität Würzburg
Am Hubland
97074 Würzburg
GERMANY
The short-time vibrational wave-packet motion in a molecule with several vibrational
degrees of freedom shows oscillatory behaviour with the characteristic
vibrational periods. For long times revivals will occur, i.e.
the wave packet restores its initial form. For
small vibrational coupling the revival times in the single modes are approximately
those of the uncoupled nuclear motion.
The fact that the revivals with respect to the single degrees of freedom
occur at different times can be used to control the
branching ratio between reaction channels by short-pulse electronic excitation at
different times. A model of the HOD molecule is used to demonstrate this
general scenario.
J. Phys. Chem., 101, 7749 (1997).
A THEORETICAL ANALYSIS OF THE TIME-RESOLVED FEMTOSECOND CARS-SPECTROSCOPY
OF I2
S. MEYER, M. SCHMITT, A. MATERNY, W. KIEFER, AND V. ENGEL
Institut für Physikalische Chemie
Universität Würzburg
Am Hubland
97074 Würzburg
GERMANY
We analyze the results of recent femtosecond CARS-experiments
on the iodine molecule (Schmitt et. al., Chem. Phys. Lett. 270 (1997) 9)
within time-dependent quantum calculations. Dependent on the delay-time between
three ultrashort pulses the third-order polarization shows oscillations which
are characteristic for the wave packet motion in the electronic ground or $B$
excited state, respectively. For the overall dependence of the CARS-signal on
the pulse delay the treatment of the vibrational degree of freedom is
sufficient. The specific decay and long time behaviour of the recorded signals
however, can only be understood if the rotational degree of freedom is taken
into account.
Chem. Phys. Lett., 281, 332 (1997).
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1998 |
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CONTINUUM RESONANCE RAMAN SCATTERING FROM ABA-TYPE MOLECULES
M. BRAUN, A. MATERNY, M. SCHMITT, W. KIEFER, AND V. ENGEL
Institut für Physikalische Chemie
Universität Würzburg
Am Hubland
97074 Würzburg
GERMANY
We theoretically investigate the continuum resonance Raman spectrum from
symmetric ABA-type molecules using a model of the pseudo triatomic molecule
X(=CH2)I. The experimental Raman spectrum of this
molecule shows pronounced excitation of the asymmetric stretching mode which
is in contrast to what is found for other molecules like H2O.
We show within time-dependent quantum calculations that this behaviour, in
principle, gives important information about the excited state potential energy
surface and in particular about the location of the reaction barrier relative
to the ground-state equilibrium configuration.
Chem. Phys. Lett., 284, 39 (1998).
PHOTOELECTRON DISTRIBUTIONS FROM FEMTOSECOND PUMP/PROBE EXCITATION WITH
CHIRPED PROBE PULSES
S. MEYER, C. MEIER, AND V. ENGEL
Institut für Physikalische Chemie
Universität Würzburg
Am Hubland
97074 Würzburg
GERMANY
We calculate photoelectron distributions obtained from the two-photon
pump/probe ionization of Na$_2$. A first ultrashort pulse prepares a
vibrational wave packet in the $(2)^1 \sum_u^+$ electronic state. The dynamics
is probed by time-delayed ionization caused by a chirped femtosecond
laser pulse. Time-resolved photoelectron spectra are determined for different
chirp parameters. It is shown that the spectra are very sensitive to the
choice of the chirp. In particular the width can be selectively modified
and the in- or outward motion of the wave packet can be distinguished by
using an appropriately chirped electric field. An analytical expression
for the photoelectron spectrum explains its dependence on the chirp parameter.
J. Chem. Phys., 108, 7631 (1998).
TIME-DEPENDENT QUANTUM CALCULATIONS ON THE NUCLEAR DYNAMICS OF LASER EXCITED
ORGANOMETALLIC MOLECULES
M. BRAUN, T. MARKERT, W. MALISCH, O. RUBNER AND V. ENGEL
Institut für Physikalische Chemie
Universität Würzburg
Am Hubland
97074 Würzburg
GERMANY
in Stereoselective Reactions of Metal-Activated Molecules - III,
edited by H. Werner and P. Schreier (Vieweg, 1998).
MOLECULAR FEMTOSECOND EXCITATION DESCRIBED WITHIN THE GAUSSIAN WAVE PACKET
APPROXIMATION
M. BRAUN, H. METIU, AND V. ENGEL
Institut für Physikalische Chemie
Universität Würzburg
Am Hubland
97074 Würzburg
GERMANY
We consider the ultra-short pulse excitation of molecules within bound
electronic states. Using UV electronic and intense pulse IR-excitation as
numerical examples we compare the approximative Gaussian wave packet (GWP)
method applied to the coherent excitation of molecules to numerically exact
solutions of the time-dependent Schrödinger equation.
The results imply that the short-time vibrational dynamics of molecules upon
femtosecond excitation can be described accurately within the GWP-method. This
especially holds for the calculation of femtosecond pump/probe signals, which
are much less sensitive to the errors of the method than the wave function is.
J. Chem. Phys., 108, 8983 (1998).
SINGLE-COLLISION CAGING OF DISSOCIATING I2: WAVE-PACKET CALCULATIONS
AND SIMULATIONS OF FEMTOSECOND SPECTRA
C. MEIER, V. ENGEL, AND A. J. BESWICK
Institut für Physikalische Chemie
Universität Würzburg
Am Hubland
97074 Würzburg
GERMANY
Based on recent experimental results on caging of I2 by collisions
with rare gas atoms at room temperature [Wan et al., J. Chem. Phys. 106,
4353 (1997)], wave packet simulations of Ar atoms colliding with I2
excited above th B-state dissociation limit by an ultrashort laser pulse, are
presented. With the dependence on the initial orientation and the collision
partners of th final vibrational state distribution of caged I2,
full time-dependent simulations of the pump-probe experiments performed by
Wan et al. for different Ar pressures were conducted. The excellent agreement
between theory and experiment confirms the single-collision caging mechanism
of Wan et al.
Chem. Phys. Lett., 287 (1998).
AN EFFECTIVE METHOD FOR THE QUANTUM MECHANICAL DESCRIPTION OF
PHOTOIONIZATION WITH ULTRASHORT INTENSE LASER PULSES
C. MEIER, V. ENGEL, AND U. MANTHE
Institut für Physikalische Chemie
Universität Würzburg
Am Hubland
97074 Würzburg
GERMANY
We present a new method to calculate photoionization
spectra which origin in the interaction of a molecule
with ultrashort strong laserpulses of arbitrary time- and
frequency shapes.
Concepts of the multi-configuration-time-dependent-Hartree (MCTDH) method,
intially developed for the propagation of multidimensional wave packets
are combined with the Fast-Fourier-Transform Split-Operator method.
The presented method allows for a fast and stable non-perturbative description
of multiphoton excitation and ionization processes in small molecules.
As an example we calculate the kinetic energy distribution of
photoelectrons from intense short pulse ionization of Na2.
J. Chem. Phys. 109, (1998).
FRAGMENTATION DYNAMICS OF FE(CO)5 UPON FEMTOSECOND EXCITATION: A
TIME-DEPENDENT STATISTICAL DESCRIPTION
O. RUBNER AND V. ENGEL
Institut für Physikalische Chemie
Universität Würzburg
Am Hubland
97074 Würzburg
GERMANY
We present a time-dependent statistical model for the multiple CO
abstraction from iron pentacarbonyl which is initiated by femtosecond
excitation. The theory is based on a classical RRKM-model. The short pulse
excitation is treated within time-dependent perturbation theory and the
fragmentation probability is calculated via the energy dependent density
of states of the parent and fragment molecules. We consider sequential
and concerted CO loss separately. Application to recent
experiments ( Ihee et. al, Chem. Phys. Lett. 281, 10, (1997))
yields good agreement with the experimental results and points towards a
sequential dissociation process.
Chem. Phys. Lett. 293, 485 (1998).
PUMP/PROBE SPECTROSCOPY OF NAI IN RARE GAS ENVIRONMENTS: A STATISTICAL
DESCRIPTION
H. DIETZ AND V. ENGEL
Institut für Physikalische Chemie
Universität Würzburg
Am Hubland
97074 Würzburg
GERMANY
A theoretical study of the femtosecond spectroscopy of the NaI predissociation
process under high pressure conditions is presented. We employ a statistical
model to mimic the pressure dependence of pump/probe signals for different
rare gases. The model uses an ensemble of trajectories to approximate the
quantum dynamics of the wave packet which is prepared by ultrashort pulse
excitation. Predissociation is treated within the Landau-Zener model for
curve crossing. Collisions are modeled as instantaneous events assuming
hard sphere scattering. The collision probability is calculated
statistically so that no adjustable parameter has to be introduced. The
results show how coherent wave-packet motion is influenced by scattering
processes and how this is reflected in femtosecond pump/probe signals.
J. Phys. Chem. A, 102, 7406 (1998).
GEMINATE RECOMBINATION PROCESSES INDUCED BY RARE GAS COLLISIONS WITH
PREDISSOCIATING NAI MOLECULES PREPARED BY FEMTOSECOND EXCITATION
H. DIETZ AND V. ENGEL
Institut für Physikalische Chemie
Universität Würzburg
Am Hubland
97074 Würzburg
GERMANY
NaI molecules predissociate after excitation with ultrashort pulses at a
peak wavelength of 320 nm. In rare gas
environments at higher pressures the process of geminate recombination
is likely to occur.
We study the dynamics of these processes under various pressure
conditions and for different collision partners.
The calculations are performed within a three-dimensional statistical
model which simulates the NaI wave-packet dynamics with classical trajectories
and the collisions via an instantaneous hard sphere scattering.
Theo. Chim. Act., 100, 199 (1998).
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1999 |
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ULTRAFAST DETECTION AND CONTROL OF MOLECULAR DYNAMICS
PHILIP ANFINRUD, REGINA DE VIVIE-RIEDLE AND VOLKER ENGEL
Institut für Physikalische Chemie
Universität Würzburg
Am Hubland
97074 Würzburg
GERMANY
Many elementary chemical and physical processes such as the breaking of a
chemical bond or the vibrational motion of atoms within a molecule take place
on a femtosecond (fs=10-15s) or picosecond (ps=10-12s)
timescale. It is now possible to monitor these events as a function of time
with temporal resolution well below 100 femtoseconds. This capability is based
upon the pump-probe technique where one optical pulse triggers a reaction and a
second delayed optical pulse probes the changes that ensue. To illustrate this
capability, the dynamics of ligand motion within a protein are presented.
Moving beyond casual observation of a reaction to active control of its outcome
requires additional experimental and theoretical effort. To illustrate the
concept of control, the effect of optical pulse duration on the vibrational
dynamics of a tri-atomic molecule are discussed. The experimental and
theoretical resources currently available are poised to make the dream of
reaction control a reality for certain molecular systems.
Proc. Natl. Acad. Sci. USA, 96 (1999).
FEMTOSECOND TIME-RESOLVED CARS SPECTROSCOPY ON BINARY GAS PHASE
MIXTURES: A THEORETICAL AND EXPERIMENTAL STUDY OF THE BENZENE/TOLUENE SYSTEM
O. RUBNER, M. SCHMITT, G. KNOPP, A. MATERNY, W. KIEFER, V. ENGEL
Institut für Physikalische Chemie
Universität Würzburg
Am Hubland
97074 Würzburg
GERMANY
Femtosecond time-resolved CARS measurements on benzene, toluene
and binary mixtures of these molecules have been performed in the gas
phase. Whereas a two-pulse excitation process prepares benzene in a single
quantum state, a vibrational wave packet is excited in toluene which gives
rise to oscillatory patterns in the CARS signal.
For the mixed system one finds beats between modes coherently excited in both
molecules. The results are analyzed using perturbative quantum calculations.
It is shown that the rotational degree of freedom is responsible for the decay
of the CARS-signal in all cases. Furthermore it is demonstrated that the
details of the excitation process have to be incorporated in the calculation
to obtain the correct amplitudes of the oscillating signals.
J. Phys. Chem. A 102, 9734 (1998).
SINGLE AND MULTIPLE COLLISION EFFECTS OBSERVED IN THE
FEMTOSECOND SPECTROSCOPY OF I$_2$-RARE GAS COLLISION COMPLEXES:
A STATISTICAL DESCRIPTION
H. DIETZ AND V. ENGEL
Institut für Physikalische Chemie
Universität Würzburg
Am Hubland
97074 Würzburg
GERMANY
We use a simple statistical model to simulate recent femtosecond experiments
on the caging dynamics of I2in high pressure rare gas environments
(Wan et al, J. Chem. Phys. 106, 4353 (1997)). The collision-induced
bound state motion in I2 and its appearance in the pump/probe
signal is perfectly reproduced in our calculation. In the case where He is
used as the I2 collision partner we find multiple collisions to be
important. The numerical results can be explained analytically under
simplified assumptions.
J. Chem. Phys., 110, 3335 (1999).
STRUCTURES IN MOLECULAR PHOTOFRAGMENT AND PHOTOELECTRON DISTRIBUTIONS FROM
TWO-PHOTON NON-RESONANT EXCITATION
A. HERWIG, M. BRAUN, AND V. ENGEL
Institut für Physikalische Chemie
Universität Würzburg
Am Hubland
97074 Würzburg
GERMANY
We regard molecular two-photon electronic transitions induced by the
interaction of two pulses with different frequencies and temporal widths.
Whereas one pulse is tuned off resonance between two vibrational levels
in an intermediate electronic state the
second one induces a transition to a dissociative
or ionic electronic state. It is shown that in both case the non-resonant
excitation induces structures in the final state distributions which are not
present for resonant excitation.
Chem. Phys. Lett., 300, 269 (1999).
SIMULATION OF FEMTOSECOND TIME-RESOLVED FOUR-WAVE MIXING EXPERIMENTS ON
I2
S. MEYER, M. SCHMITT, A. MATERNY, W. KIEFER, AND V. ENGEL
Institut für Physikalische Chemie
Universität Würzburg
Am Hubland
97074 Würzburg
GERMANY
Quantum calculations on time-resolved coherent
spectroscopy of gas-phase I2 samples are
presented. The total polarization induced in the
molecules depends on the delay-time between the
applied laser pulses. Different contributions reflect
the nuclear dynamics in the ground or an excited
electronic state. Dependent on the pulse delay-time
on one hand and the wavelengths employed on
the other, the signal reflects dynamics in a single
or in both electronic states. In the case that several
initially populated vibrational states contribute to
the signal, special care has to be taken of the
experimental detection process.
Chem. Phys. Lett., 301, 248 (1999).
A NOVEL MIXED QUANTUM/CLASSICAL METHOD FOR THE DYNAMICS OF MOLECULES IN A GAS
ENVIRONMENT WITH AN APPLICATION TO FEMTOSECOND TIME-RESOLVED
SPECTROSCOPY
V. ERMOSHIN, A. K. KAZANSKY*, AND V. ENGEL
Institut für Physikalische Chemie
Universität Würzburg
Am Hubland
97074 Würzburg
GERMANY
*Institute of Physics
The University of St.Petersburg
198904 St.Petersburg
RUSSIA
A mixed quantum/classical method is proposed to describe
the dynamical behaviour of a diatomic molecule in a gas environment.
The vibrational coordinate is treated quantum-mechanically and all other
degrees of freedom classically.
Within the present approach the classical equations for the rotational
motion have no singularities. A simplectic algorithm is used for the
propagation. As an application we treat collisions of rare gas
atoms with I2 excited by femtosecond pulses. The decay of
the pump-probe signal via the thermal average over rotational states and by
collisions with the buffer gas is considered.
Chem. Phys. Lett., 302, 20 (1999).
A CASSCF/MR-CCI STUDY OF THE EXCITED STATES OF FE(CO)5
O. RUBNER, V. ENGEL,
Institut für Physikalische Chemie
Universität Würzburg
Am Hubland
97074 Würzburg
GERMANY
M. D. HACHEY AND C. DANIEL
Laboratoire de Chimie Quantique
UMR 7551 du CNRS et de l'Université Louis Pasteur
Institut le Bel
4 Rue Blaise Pascal
F-67000 Strasbourg, France
CASSCF/MR-CCI calculations have been performed to determine excited states
of Fe(CO)5 in the VUV region as well as the corresponding dipole
moments. In spite of the high density of states between 25 000 and 45 000
cm-1, only three allowed transitions, with rather high oscillator
strengths, 1A'1 -> a 1E',
1A'1 -> 1A"2,
1A'1 -> b 1E' are present. The
a 1E' state is metal-centered (3d to 3d excitation) whereas the
1A"2 and b 1E' correspond to 3d to
*CO excitations. The
importance of these states for dissociation processes is discussed.
Chem. Phys. Lett., 302, 489 (1999).
CONSTRUCTION OF A POTENTIAL ENERGY SURFACE FOR
MOLECULAR DYNAMICS STUDIES OF METHANE ADSORBED IN ZEOLITES
V. ERMOSHIN AND V. ENGEL
Institut für Physikalische Chemie
Universität Würzburg
Am Hubland
97074 Würzburg
GERMANY
A potential for the interaction of methane molecules with the framework atoms
of siliceous zeolites is developped empirically. The CH4 molecule
is considered in the spherical approximation. The interaction potential
includes repulsion, dispersion, and induction energy. It is shown that
even if the effective charges on the framework oxygen and silicon atoms
are large, the contribution of the induction energy
is weak in comparison with those of dispersion and repulsion energies
due to compensation effects. The calculated parameters of
a Lennard-Jones potential are compared with those
used in previous MD calculations. As an application we calculate
diffusion coefficients and the heat of adsorption of methane in
silicalite which can be compared to available experimental data.
J. Phys. Chem. A 103, 5116 (1999).
DETERMINATION OF WAVE-PACKET DYNAMICS BY FEMTOSECOND TIME-RESOLVED
PUMP-DUMP-PROBE AND FOUR-WAVE MIXING TECHNIQUES
T. CHEN, V. ENGEL, M. HEID, W. KIEFER, G. KNOPP, A. MATERNY, S. MEYER,
R. PAUSCH, M. SCHMITT, H. SCHWOERER, AND T. SIEBERT.
Institut für Physikalische Chemie
Universität Würzburg
Am Hubland
97074 Würzburg
GERMANY
J. Molec. Struc. 480-481, 33 (1999).
QUANTUM-CLASSICAL MOLECULAR DYNAMICS SIMULATION OF FEMTOSECOND SPECTROSCOPY ON
I2 IN INERT GASES: MECHANISMS FOR THE DECAY OF PUMP-PROBE SIGNALS
V. ERMOSHIN, A. K. KAZANSKY*, AND V. ENGEL.
Institut für Physikalische Chemie
Universität Würzburg
Am Hubland
97074 Würzburg
GERMANY
*Institute of Physics
The University of St.Petersburg
198904 St.Petersburg
RUSSIA
A mixed quantum-classical method is proposed to describe the
dynamical behavior of a diatomic
molecule in a gas environment.
The vibrational coordinate is treated quantum-mechanically and all
other degrees of freedom classically. Within the present approach
the classical equations for the
rotational motion have no singularities. A symplectic, energy
conserving and time-reversible
algorithm is used for the propagation. As an application we treat
the dynamics of I2 molecules
excited by femtosecond laser pulses moving in collision-free
and high pressure rare-gas
environments. For freely rotating I2 molecules, the thermal
average over rotational states leads to the
decay of the pump-probe signal. For I2 in inert gases,
we show that dephasing by collisions with the
buffer gas is a weak effect in comparison with the decay of the
signal due to the anharmonicity of
the potential energy curve. Therefore the oscillating structure
of the pump-probe signals depends
weakly on the mass and the pressure of the solvent,
in agreement with experimental data.
J. Chem. Phys. 111, 7807 (1999).
CLASSICAL SIMULATIONS ON THE PUMP-PROBE SPECTROSCOPY OF I2
ENCAPSULATED IN DDR POROSIL
V. A. ERMOSHIN, G. FLACHENECKER, A. MATERNY, AND V. ENGEL
Institut für Physikalische Chemie
Universität Würzburg
Am Hubland
97074 Würzburg
GERMANY
We present molecular dynamics simulations of transients
obtained by femtosecond time-resolved spectroscopy on I2
embedded in porosil deca-dodecasil 3R. The calculated
signals are compared with recent experimental data obtained in our
laboratory. It is shown that the pump-probe signals contain three
contributions originating from D´<-A´, E<-B, and f<-B
optical transitions in I2 .
The influence of predissociation and vibrational
relaxation on the pump-probe signals at different
wavelengths is discussed.
Chem. Phys. Lett. 311, 146 (1999).
ON THE DECONVOLUTION OF THE TEMPORAL WIDTH OF LASER PULSES FROM
PUMP-PROBE SIGNALS
N.E. HENRIKSEN
Department of Chemistry
Technical University of Denmark
DTU 207
DK 2800 Lyngby, Denmark
V. ENGEL
Institut für Physikalische Chemie
Universität Würzburg
Am Hubland
97074 Würzburg, Germany
We consider pump-probe frequency integrated fluorescence signals.
These transients, in general, arise from wave packets where the
intrinsic dynamics and the response to the pump pulse is
entangled in a complicated way. The signal from an infinitely short
-pump pulse is the only
exception. However, if the pump-probe signal is integrated over
the frequencies of the pump laser, the signal can be expressed
as a convolution of the pulse envelope with the signal obtained from a
-pulse excitation. We consider
the deconvolution of the pulse envelope and demonstrate that the signal
corresponding to a -pump pulse
can be recovered exactly from signals using pump pulses of finite
temporal width. Numerical examples are presented for asymptotic
fragment detection in the reaction, ICN->I+CN.
J. Chem. Phys. 111, 10469 (1999).
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2000 |
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FEMTOSECOND TIME-RESOLVED CARS AND DFWM SPECTROSCOPY ON GAS PHASE
I2: A WAVEPACKET DESCRIPTION
S. MEYER AND V. ENGEL
Institut für Physikalische Chemie
Universität Würzburg
Am Hubland
97074 Würzburg
GERMANY
Femtosecond time-resolved CARS (coherent anti-Stokes Raman scattering)
and DFWM
(degenerate four wave mixing) experiments on gas phase iodine
molecules are analysed within time dependent wave-packet calculations.
The recorded signals are interpreted using the
ro-vibrational quantum dynamics in different electronic states
of I2.
Thereby
various contributions to the induced third-order polarization
are related to overlap integrals between wave packets created
by absorption and emission processes. This allows for an
intuitive interpretation of the temporal variations in the signals.
J. Raman Spec. 31, 0000 (2000).
MAPPING OF WAVE PACKETS IN DIRECT FRAGMENTATION VIA
PUMP-PROBE FREQUENCY INTEGRATED FLUORESCENCE SPECTROSCOPY
V. ENGEL
Institut für Physikalische Chemie
Universität Würzburg
Am Hubland
97074 Würzburg, Germany
N.E. HENRIKSEN
Department of Chemistry
Technical University of Denmark
DTU 207
DK 2800 Lyngby, Denmark
We consider femtosecond excitation of a molecule to a dissociative
electronic state. The quantum dynamics is recorded via
delayed excitation to
a higher electronic state and measurement of the total fluorescence
from this state detected as a function of delay time. It is shown that
the signal can be used to determine the probability density distribution
of the outgoing wave packet describing the fragmentation.
This, in particular, applies to the case of fragment detection
since then the time-dependent signal directly measures the probability
flux at a fixed value of the dissociation coordinate.
Numerical examples illustrate the procedure.
J. Chem. Phys. 112, 106 (2000).
THEORETICAL ANALYSIS OF FEMTOSECOND EXCITATION AND FRAGMENTATION
DYNAMICS OF FE(CO)5
O. RUBNER, T. BAUMERT, M. BERGT, B. KIEFER, G. GERBER, AND V. ENGEL
Institut für Physikalische Chemie
Universität Würzburg
Am Hubland
97074 Würzburg, Germany
We present the application of a time-dependent model to
Fe(CO)5 dissociation after femtosecond excitation to understand
transient signals obtained in recent pump/probe experiments.
The calculation of
time-dependent energy distributions allows us to
interprete the decay behaviour of the various fragments.
The results indicate that at least two dissociation
pathways have to be considered to explain the transient signals
obtained for the different fragments.
Furthermore it is shown that within each of the discussed
pathways concerted and sequential
fragmentation compete.
Chem. Phys. Lett. 316, 583 (2000).
ON THE MECHANISM OF STRONG-FIELD DOUBLE PHOTOIONIZATION IN THE
HELIUM ATOM
M. LEIN,a,b E.K.U. GROSS,b and V. ENGELa
aInstitut für Physikalische Chemie
Universität Würzburg
Am Hubland
97074 Würzburg, Germany
bInstitut für Theoretische Physik
Universität Würzburg
Am Hubland
97074 Würzburg, Germany
We perform time-dependent quantum calculations on the ionization
dynamics of helium in intense ultrashort laser fields. A linear model
of the atom is used to extract single and double-ionization yields for
a 780 nm excitation. For an intensity of 1015 W cm-2
clear evidence is found for a direct double-ionization
process which is caused by rescattering of an electron from the core
in the presence of the field. A momentum-space analysis of the wave
functions shows that in this case both electrons are preferentially emitted
in the same direction and with different momenta.
J. Phys. B 33, 433 (2000).
HYBRID QUANTUM/CLASSICAL STUDY OF ICN IN AN Ar MATRIX:
PHOTOFRAGMENTATION AND CAGE EXIT
S. FERNANDEZ ALBERTI,a J. ECHAVE,a
V. ENGEL,b N. HALBERSTADT,c AND J. A. BESWICKc
aUniversidad Nacional de Quilmes, S. Peņa 180, 1876 Bernal, Argentina
bInstitut für Physikalische Chemie,
Universität Würzburg, Am Hubland, D-97074 Würzburg, Germany
cIRSAMC, Université Paul Sabatier, 118 route de Narbonne, 31062 Toulouse, France
The à continuum photoexcitation of ICN
in an Ar matrix is studied using an implementation of
the molecular dynamics with quantum transitions method of Tully.
Five excited electronic potential energy surfaces of the ICN
molecule,
30+,
11(A',A''),
31(A',A''),
as well as its ground state, are included in these calculations.
The couplings between electronic states at large I–CN internuclear
distances are modeled using
a diatomic in molecules treatment of the mixing of the
different spin-orbit states of iodine induced
by the Ar atoms. The electronic motion, as well as the I–CN distance and
the corresponding bending angle, are treated quantum mechanically
using wave-packet techniques. The
rotation and translation of the ICN molecule in the Ar cage are
treated classically, as well as the
motion of the Ar atoms. In contrast with previous calculations,
in which all nuclear degrees of freedom were
treated classically, we found a 2% of CN cage exit during the
first 0.5 ps of the dynamics.
J. Chem. Phys. 113, 1027 (2000).
CHARACTERIZATION OF NUCLEAR WAVE PACKETS DESCRIBING MOLECULAR PHOTODISSOCIATION
M. LEIN,a,b M. ERDMANN,a and V. ENGELa
aInstitut für Physikalische Chemie
Universität Würzburg
Am Hubland
97074 Würzburg, Germany
bInstitut für Theoretische Physik
Universität Würzburg
Am Hubland
97074 Würzburg, Germany
A bound-to-free transition initiated by femtosecond excitation of diatomic
molecules results in photofragments with a distribution of kinetic energies.
A measurement of the kinetic-energy distribution
yields the modulus squared of the asymptotic momentum-space wave
packet prepared in the laser excitation process. On the other hand, the
coordinate-space density of the wave packet entering the interaction-free
region can be determined from pump-probe integrated fluorescence
spectroscopy. We provide several numerical examples to show that this
information can be used to determine the phase of the asymptotic wave
packet so that this particular quantum-mechanical
wave function can be characterized completely.
To achieve this aim we
use an iteration scheme (Gerchberg-Saxton algorithm)
which does not require any further information
about the system or the laser pulses.
J. Chem. Phys. 113, 3609 (2000).
MOLECULAR DYNAMICS SIMULATION OF FEMTOSECOND PUMP-PROBE EXPERIMENTS
ON I2 IN Ar ENVIRONMENTS
V. A. ERMOSHIN and V. ENGEL
Institut für Physikalische Chemie
Universität Würzburg
Am Hubland
97074 Würzburg, Germany
We present the application of classical molecular dynamics methods
to the simulation of femtosecond time-resolved experiments.
Choosing I2 in Ar as an example, we consider two cases of laser
excitation processes:
i) a 496 nm excitation from the ground electronic state
to the excited B-state prepares a wave packet within the dissociation
continuum.
Collisions between dissociating molecules and Ar atoms lead to caging of
I2;
ii) a 620 nm pulse creates a coherent superposition of
bound vibrational states and the interaction with Ar leads to a
dephasing
of the wave packet. We calculate pump-probe signals which are
in excellent agreement with experiment. The purely classical simulation
gives valuable insight into the dynamical processes which are
reflected in the time-resolved signals.
Z. Phys. Chemie 214, 1265 (2000).
WAVE PACKET DYNAMICS IN DIFFERENT ELECTRONIC STATES INVESTIGATED
BY FEMTOSECOND TIME-RESOLVED FOUR-WAVE-MIXING SPECTROSCOPY
A. MATERNY, T. CHEN, M. SCHMITT, T. SIEBERT, A. VIERHEILIG, V. ENGEL, W. KIEFER
Institut für Physikalische Chemie
Universität Würzburg
Am Hubland
97074 Würzburg, Germany
This paper reviews results on wave packet dynamics investigated by means of femtosecond
time-resolved four-wave-mixing (FWM)
spectroscopy. First, it is shown that by making use of the various degrees of freedom
which are offered by this technique information about
molecular dynamics on different potential-energy surfaces can be accessed and separated
from each other. By varying the timing,
polarization, and wavelengths of the laser pulses as well as the wavelength of the
detection window for the FWM signal, different dynamics
are coherently excited and probed by the nonlinear spectroscopy. As a model system
we use iodine in the gas phase. These techniques are
then applied to more-complex molecules (gas phase: benzene, toluene, a binary
mixture of benzene and toluene; solid state: polymers of
diacetylene matrix-isolated in single crystals of monomer molecules).
Here, ground-state dynamics are investigated first without any
involvement of electronically excited states and then in electronic
resonance to an absorption transition in the investigated molecules.
Signal modulations result which are due to wave packet motion as well
as polarization beats between modes in different molecules. Phase
and intensity changes yield information about intramolecular vibrational
energy redistribution, population decay (T1), phase relaxation
(T2), and coherence times.
Appl. Phys. B - Lasers and Optics 71, 299 (2000).
NON-PERTURBATIVE WAVE-PACKET CALCULATIONS OF TIME-RESOLVED FOUR-WAVE-MIXING SIGNALS
S. MEYER, V. ENGEL
Institut für Physikalische Chemie
Universität Würzburg
Am Hubland
97074 Würzburg, Germany
We extend a method proposed by Seidner et al. [J. Chem. Phys. 103, 3998 (1945)]
to extract directional terms from a time-dependent
molecular polarization to obtain time-resolved four-wave-mixing (FWM) signals.
Instead of employing perturbation theory, the total
polarization induced in the molecular sample by the interaction with several
femtosecond laser pulses is determined by solving the
time-dependent Schrödinger equation for the nuclear dynamics in coupled
electronic states numerically exact. Repeating the calculation
for several combinations of relative phases of the involved fields leads
to a linear system of equations for the contributions emitted in
different directions. Gas-phase I2 molecules serve as a numerical example.
Appl. Phys. B - Lasers and Optics 71, 293 (2000).
OSCILLATORY PUMP-PROBE SIGNALS FROM DELOCALIZED WAVE PACKETS
V. A. ERMOSHIN and V. ENGEL
Institut für Physikalische Chemie
Universität Würzburg
Am Hubland
97074 Würzburg, Germany
C. MEIER
Laboratoire de Collisions, Agrégats et Reactivité, IRSAMC,
Université Paul Sabatier,
31062 Toulouse, France
We calculate pump-probe signals obtained from femtosecond excitation of
I2 molecules. It is shown that, depending on the parameters of
the pump laser-pulse, the signals can exhibit a regular oscillatory behavior
although the vibrational wave packet which is produced in the
pump process, is delocalized over the entire classically allowed region.
The respective structures can be related to temporal variations of an
enhanced probability density which, in turn, are related to the underlying
classical motion. We further demonstrate that the use of
time-resolved photoelectron spectroscopy allows for a characterization of the
structures which appear in the time-dependent wave function.
J. Chem. Phys. 113, 5770 (2000).
COLLISION-INDUCED BOUND STATE MOTION IN I2. A CLASSICAL
MOLECULAR DYNAMICS
V. A. ERMOSHIN and V. ENGEL
Institut für Physikalische Chemie
Universität Würzburg
Am Hubland
97074 Würzburg, Germany
C. MEIER
Laboratoire de Collisions,
Agrégate et Reactivité,
IRSAMC
Université Paul Sabatier
31062 Toulouse, France
We use three-dimensional classical molecular dynamics to simulate
femtosecond time-resolved experiments on the caging dynamics of I2
in rare gas environments [Wan et al., J. Chem. Phys. 106, 4353 (1997)].
The calculated pump-probe signals are in excellent agreement
with experiment. Prominent oscillatory structures as observed in the
pump-probe signals are interpreted in terms of caustics which appear
in the classical bound state dynamics of the caged molecules.
The results confirm conclusions based on a simple statistical model which
treats the collisions as random perturbations involving hard sphere scattering.
J. Chem. Phys. 113, 6585 (2000).
PULSE-WIDTH AND ISOTOPE EFFECTS IN FEMTOSECOND-PULSE STRONG-FIELD
DISSOCIATION OF H2+ and D2+
C. TRUMP,1 H. ROTTKE,1 M. WITTMANN,1 G. KORN,1
and W. SANDNER1,2
1Max-Born-Institut, Max-Born-Straße 2a, D-12489 Berlin, Germany
2Technische Universität Berlin, Hardenbergstraße 36,
D-10623 Berlin, Germany
M. LEIN3,4 and V. ENGEL3
3Institut für Physikalische Chemie,
Universität Würzburg,
Am Hubland,
97074 Würzburg, Germany
4Institut für Theoretische Physik
Universität Würzburg
Am Hubland
97074 Würzburg, Germany
Using a high-resolution setup, we investigate the fragmentation
of H2 and D2 in intense 790 nm laser pulses with
pulse widths of 28 fs and 80 fs.
The measured H+/D+ kinetic energies
resulting from effective two-photon dissociation of
H2+/D2+
show a clear dependence on the pulse width and on the isotope.
One-dimensional calculations reproduce these effects and show that
they mainly originate from the H2+/D2+
dissociation dynamics.
Further, we find that the initial ionic state
before dissociation is not simply
given as a Franck-Condon
distribution over unperturbed ionic states. Rather it is a
complex mixture of initial wave packets
(covering, however, a small energy range) created by multiphoton
ionization of H2 and D2 at various times during the action of
the pulse.
Phys. Rev. A 62, 063402 (2000).
PHASE-ENERGY APPROACH TO COLLISION-INDUCED VIBRATIONAL RELAXATION
A. K. KAZANSKY
Fock Institute of Physics
The University of St. Petersburg
198504 St. Petersburg, Russia
V. A. ERMOSHIN and V. ENGEL
Institut für Physikalische Chemie
Universität Würzburg
Am Hubland
97074 Würzburg, Germany
A representation of the diatomic vibrational distribution function in energy
and phase variables yields a transparent physical picture of
collision-induced vibrational relaxation. In this way, the energy- and
phase-relaxation processes can be separated. A simple
parametrization of the phase distribution allows for the extraction of
the dephasing time and the reconstruction of autocorrelation
functions and femtosecond pump-probe signals. It is shown that the
dephasing time can be derived from the Fourier spectrum of the
pump-probe signal in favorable cases.
J. Chem. Phys. 113, 8865 (2000).
INTENSE-FIELD DOUBLE IONIZATION OF HELIUM: IDENTIFYING THE MECHANISM
M. LEIN,a,b E.K.U. GROSS,b and V. ENGELa
aInstitut für Physikalische Chemie
Universität Würzburg
Am Hubland
97074 Würzburg, Germany
bInstitut für Theoretische Physik
Universität Würzburg
Am Hubland
97074 Würzburg, Germany
We present quantum mechanical calculations of the electron
and ion momentum distributions
following double ionization of a one-dimensional helium atom
by ultrashort laser pulses (780 nm)
at various intensities.
The two-electron momentum distributions
exhibit a clear transition from non-sequential
to sequential double ionization.
We provide strong evidence that rescattering is responsible
for non-sequential ionization by
calculating the momentum spectrum of the He2+ recoil ions -
which we find in excellent agreement with recent experiments -
and by analysing the electronic center-of-mass motion
via Wigner transforms.
Phys. Rev. Lett. 85, 4707 (2000).
STOCHASTIC TRAJECTORY SIMULATION OF FEMTOSECOND PUMP-PROBE SPECTROSCOPY
V. A. ERMOSHIN and V. ENGEL
Institut für Physikalische Chemie
Universität Würzburg
Am Hubland
97074 Würzburg, Germany
Regarding the transient spectroscopy of I2 in rare-gas environments,
it is shown how to calculate femtosecond pump-probe signals with the
help of
the stochastic Langevin equation.
The method is very fast on the cost of the introduction of an additional
parameter which describes the energy flow between the system and
environment.
The stochastic trajectory approach is tested by comparison with
full-scale molecular dynamics calculations
and good agreement is obtained for the signals.
The stochastic trajectory simulation can be applied
for the interpretation of experimental
signals and adjustment of external parameters in
theoretical models.
Chem. Phys. Lett. 332, 110 (2000).
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2001 |
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PHASE-SPACE ANALYSIS OF DOUBLE IONIZATION
M. LEIN,a,b V. ENGEL,a and E.K.U. GROSSb
aInstitut für Physikalische Chemie
Universität Würzburg
Am Hubland
97074 Würzburg, Germany
bInstitut für Theoretische Physik
Universität Würzburg
Am Hubland
97074 Würzburg, Germany
We use the Wigner transformation to study
the electronic center-of-mass motion in phase-space
for double ionization in a
strong laser field. The rescattering
mechanism is clearly visible in the evolution of
the fully correlated two-electron system.
In a mean-field calculation, on the other hand,
the signatures of
rescattering are missing. Some properties of the Wigner function
in two-particle systems are reported.
Opt. Express 8, 411 (2000).
DISCRETE PEAKS IN ABOVE-THRESHOLD DOUBLE-IONIZATION SPECTRA
M. LEIN,a,b E.K.U. GROSS,b and V. ENGELa
aInstitut für Physikalische Chemie
Universität Würzburg
Am Hubland
97074 Würzburg, Germany
bInstitut für Theoretische Physik
Universität Würzburg
Am Hubland
97074 Würzburg, Germany
Quantum mechanical calculations
of multiphoton double ionization
by intense laser pulses show that the total-kinetic-energy
spectrum of the photoelectrons consists of peaks separated by
the photon energy, analogous to ordinary above-threshold
ionization. Related structures appear
in the two-electron and in the recoil-ion
momentum distribution. We propose a method to extract the
total-kinetic-energy spectrum from the experimental recoil-ion
spectrum.
Phys. Rev. A 64, 023406 (2001).
MANIFESTATION OF PREDISSOCIATION AND VIBRATIONAL RELAXATION IN
FEMTOSECOND PUMP-PROBE SIGNALS: A THEORETICAL ANALYSIS
V. A. ERMOSHIN and V. ENGEL
Institut für Physikalische Chemie
Universität Würzburg
Am Hubland
97074 Würzburg, Germany
Classical molecular dynamics is used to study
vibrational relaxation and predissociation
processes in I2 embedded in a rare-gas environment.
It is shown that the simultaneous effect of the two mechanisms
determines the deviation of experimental
pump-probe signals from a single-exponential decay [Lienau and Zewail,
J. Phys. Chem. 100 (1996) 18629].
Chem. Phys. Lett. 336, 262 (2001).
reverse
chron.