Laser Chemistry Emeritus Group - Max Planck Institut für Quantenoptik

Scientific Reports
Laser Chemistry Emeritus Group
325
Karl-Ludwig Kompa
Max-Planck-Institut für Quantenoptik · Progress Report 2009/2010
326
Laser Chemistry Emeritus Group
Senior Scientists/
PostDocs
W. Christen
W. Fuß (retired in 2008)
M. Lezius
F. Rebentrost (retired in 2008,
now part time contract)
W. Schmid (retired in 2009)
H. Schröder (retired in 2009,
now part time contract)
H. Skenderović
E. Torres
Doctoral Candidates
P. Lang
K. Karki (until 5/2010)
Visiting Scholars
(at MPQ longer than one month)
N. Johnson (Kansas State
University, Fulbright
Fellow)
External Scientific
Member
R.D. Levine (Hebrew
University, Jerusalem)
Technical/
Administrative Staff
K. Bauer
B. Bohm
W. Ritt
W. Zeiher (LMU, Munich,
working student)
Group Members
Laser Chemistry Emeritus Group
Summary of Scientific Activities
The activities of the Laser Chemistry Division have focussed
on the experimental and theoretical investigation of ultrafast dynamics in and coherent control of small to medium
sized molecules in intense laser fields. Our particular research
fields have been associated with
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Molecular computing
Ultrafast intramolecular dynamics
Coherent electron dynamics
Time resolved spectroscopy of highly excited states
Electron rescattering spectroscopy and electron
holography
Ion microscopy for spatially resolved ionization
spectroscopy
Cluster-surface collision induced desorption processes
Laser collision spectroscopy
Nonlinear optical response of semiconductor surfaces
Our achievements during the last two years include the
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Demonstration of high fidelity coherent control of
molecules
Development of ion microscopy for strong field
spectroscopy
Worlds first XUV-pump XUV-probe experiments on
molecules
Experimental demonstration of electron wavepacket
holography
Coherent control of electron localisation
Cluster-collision desorption of biomolecules like
oligopeptides
Combination of supercritical fluid coexpansion
techniques with time resolved electron spectroscopy
Laser-collision spectroscopy of sodium-methane
collisions
We have reached these achievements by numerous national
and international collaborations with many friends and
colleagues. We thank them for the opportunity to join our
forces and to break new ground in so many areas.
Overview
For a long time photon-driven chemistry has suffered from
three facts: (a) Light fields have been too weak to strongly
affect the energy flow in molecules, (b) the molecular
response to light has been too slow to avoid non-adiabaticity,
(c) using the light phase for directing chemical processes
appeared to be complicated and tedious. This has led to
requests for higher electromagnetic fields, a time resolution
approaching molecular time scales, and further development
of phase controlled laser matter interaction. Most of these
requirements can now be met by advanced laser sources and
modern spectroscopy. Laboratory size lasers can provide
Summary of Scientific Activities
327
electric fields approaching or even exceeding intra-atomic
fields. Laser pulse durations are available in the fs or even
sub-fs regime, e.g. in the timescale of intramolecular electron
dynamics. Last but not least, spectral phase control schemes
have been very successfully demonstrated with small
molecules. The available technologies provide now a complete
toolbox to control chemistry on the nanoscopic scale. In this
context it is then possible to envision, that molecules could
be used for information technology like microscopic devices;
they could be employed as switches, storage units, or even in
a more generalized sense, as molecular processors. Of course,
to achieve this goal many problems remain to be overcome:
For example, of more technical nature is perhaps the fidelity
of switching, e.g. contrast ratio, quantum yield, reset, thermal
load and fading. Even more important is the achievement
of diabatic (or deterministic) state changes in molecules
when compared to competing adiabatic (indeterministic)
ones. However, quite considerable progress has been made
in this field by the application of designed laser pulse control
instead of unshaped broadband laser pulses (Brumer
and Shapiro, Judson and Rabitz, Tannor, Rice, Kosloff,
see also the six Ringberg Symposia “Coherent Control, Theory and Experiment” M. Motzkus, K.-L. Kompa,
R. de Vivie-Riedle).
Over the year 2010 the experimental activities of the Laser
Chemistry Division will be fading out, and we present
here an overview over our recent research and how these
projects will be transferred to our collaborators and friends.
We have been actively taking part in the investigation of a
large variety of aspects regarding photo-induced dynamics
and control in atoms and molecules. As laser pulses are
becoming shorter and shorter, new fields and opportunities
emerge, and it has finally become possible to interrogate
the interplay between electronic transitions and nuclear
dynamics at unprecedented time-scales. For such studies
we have developed and extended some new and interesting
spectroscopic techniques, with the advantage to exclude
focal averaging effects in laser-matter interaction. We have
also demonstrated that coherent control can steer nuclear,
electronic and combined dynamics, and that the outcome of
photo-induced chemical reactions can be largely influenced
and with high fidelity. With the possibility to apply the next
generation of phase stable amplified few-cycle lasers in our
long-lasting and fruitful collaboration with F. Krausz and
M. Kling we have retrieved molecular information on subfemtosecond time-scales, which is opening up the fascinating
opportunity to elucidate electron dynamics while nuclei are
still at rest. The lasers that have been applied by the group
to various molecular and atomic targets have been ranging
from the intense XUV of large scale free electron lasers to
phase-controlled mid-IR pulses from table-top systems. This
also means that we have investigated molecular excitation
from core level up to purely vibrational transitions. We
have collected such data with state-of-the-art spectroscopic
techniques like COLTRIMS, VMI, Ion Microscopy and
precision NIR spectrometry.
Max-Planck-Institut für Quantenoptik · Progress Report 2009/2010
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Laser Chemistry Emeritus Group
Facilities and Collaborations
Most of our projects draw on several complementary laser
sources that have been pursued to achieve 5 to 10-fs time
resolved UV and XUV pulses. Our primary system consists
of two frequency doubled and tunable non-collinear optical
parametric amplifiers (NOPA) which are pumped by a
commercial Ti:sapphire system (CLARK 2010). In this
set-up we currently operate at pulse durations of about
20 fs. Further compression techniques have been developed
by our collaborators of the Institute for Biomolecular
Optics of the LMU and can be applied, if necessary [1].
This laser system has been recently transferred to the
Technische Universität München and is now hosted by
R. Kienberger. A second Ti:sapphire system is based on a
Spitfire regenerative amplifier. This powerful (2 mJ, 40 fs)
laser system can be compressed using plasma channeling
and chirped mirrors to about 0.5 mJ, 10 fs. Third, fifth and
higher harmonics of 800 nm can be generated in a gas target,
and resulting pulse durations comparable to the driver pulse
duration can be achieved [2]. This system will in future be
hosted by F. Krausz and coworkers. For NIR wavelengths,
we have developed a powerful femtosecond system which
is tunable in the IR (1600-3600 nm) and based on optical
parametric amplification. This system has turned out to be
particularly useful for our molecular computing experiments
[3]. This laser system will now be hosted by M. Motzkus
at the University of Heidelberg. We are very glad that
even more advanced femto- and attosecond sources within
MAP (13 nm, 80 as in the group of F. Krausz) have been
made temporarily available to us in the past years for joint
experiments. In the extended UV range we also had unique
access to the extremely bright source at FLASH, DESY
(13-50 nm, 10-50 fs, tunable, 100 µJ, [4]), in a collaboration
with J. Ullrich from the MPI for Nuclear Physics. The
deliberate selection of the EUV pulse length between 100 as
and several fs permits optimization between the necessary
temporal resolution and the energetic resolution needed to
resolve excited electronic state dynamics.
The fundamental recipe of our experiments has been
always similar. Selected cold molecules are pumped by
one or several photons into selected excited states and the
subsequent intramolecular dynamics is then traced via
photo-ionization by a second, time delayed, photon. The
resulting time dependent photo-electron (and photoion)
energy or momentum distributions are then detected
with high resolution spectrometry, in most cases homebuilt. Coincidence detection as well as angular resolution
employing velocity map imaging and reaction microscopy
techniques [5] have been realized recently.
A very interesting aspect concerns our molecular target
preparation techniques. Many complex molecules are
expected to show ultrafast internal dynamics which would
be interesting to organic chemistry, biochemistry or
biology. Unfortunately biologically relevant molecules are
quite complex and sensitive, which causes several common
Summary of Scientific Activities
problems to photo-electron spectroscopy. They often have a
low vapour pressure and behave sticky to conducting surfaces.
Moreover, they are easily destroyed upon heating. Therefore
we have developed a differentially pumped molecular beam
that is capable to operate via the supercritical fluid (SCF)
expansion technique [6]. Supercritical fluids are well known
for their ability to dissolve many organic molecules. In
supercritical co-expansion the achievable cooling rates
are extremely high, and very cold (mK) complex organic
molecules can be injected into the laser focus with a high
density beam, without the risk of pyrolitic damage. The
resulting molecular beam transits the particle spectrometer
without surface collisions and in addition in many cases the
carrier gas carbon dioxide remains unaffected by the laser
irradiation because the ionization energy of CO2 is much
higher than for organic molecules. In such an experiment
surface charging can be avoided without heating of the
spectrometer, and vibrationally cold bio-organic molecules
can be investigated with multi-photon time-resolved
spectroscopy.
In an other successful approach in collaboration with
M. Dürr (University Appl. Sci. Esslingen) we have further
developed our cluster induced desorption method [7], which
has become a reliable tool to raise ample amounts of charged
bio-molecules (oligopeptides) into the gas phase. Owing to
the soft desorption mechanism and in combination with
evaporative cooling molecular fragmentation can be well
prevented. The resulting charged molecules can be further
processed with ion trap technologies or with ion optical
means. Thus, this class of molecules is now available for
further experiments.
In the past years we have initiated over 20 collaborations
with internal and external groups to extend our scope and
possibilities. These collaborations have not only been driven
by the opportunity to access unique research facilities and
experiments, but by our common scientific interest.
Exploring Molecular Computing using
Infrared Spectroscopy
In general one is free to chose to address either nuclear
or electronic dynamics in molecules for coherent control.
However, for the time being we have decided to explore rather
vibrational states in polyatomic molecules for molecular
computing strategies, and have left manipulating electronic
states for the future. We already know that vibrational
states are well localized and that they are sufficiently longlived for efficient manipulation with high fidelity. Moreover,
these states can be distinctively coupled and switched in
various, interesting ways. Thus we are convinced that they
are suitable for an extension of photon driven chemistry in
the framework of molecular information and information
processing. In fact, there is now a long history of that in
our division. In this context it should be mentioned that
the number of bit-carrying states that could be encoded by
Laser Chemistry Emeritus Group
amplitude and phase into a simple diatomic system could
be large enough to carry a million bits within one molecule.
Thus the advancement of vibrational laser control is not only
a chance for chemistry but also for molecular informatics on
a massively parallel basis. We already assume that we can
control nuclear motion much better than electronic motion.
The question remains, which nuclear motions do we wish
to address under such aspects. Typical systems that we
have chosen for our investigations fall into the category of
medium-size metal-organic compounds [3], like W(CO)6.
Such molecules have several degrees of freedom with well
known coupling and symmetries. Spectrally, we investigate
such molecules in the mid-IR region, which guarantees an
optimum signal-to-noise ratio at room temperature. The
observables that we choose for molecular logic are selected
from a basic set of vibrational levels, which we are able to
optically distinguish, read out and manipulate. Our major
goal in this area is to provide some basic understanding
of how to use the complexity of molecules to dig out their
functionalities via optical addressing and read out. Within
this context we have developed several original strategies to
manipulate and to control laser pulses in the mid-IR. The
fidelity that we can achieve with our molecular processors is
impressive: The corresponding population dynamics shows
that we begin with all of the population in the initial ground
state v=0 and after the laser pulse 99 % of the population
has been transferred to v=2 state. As a next step we plan
to implement improved spectral shaping schemes based on
acousto-optic shapers in the mid-IR in a collaboration with
M. Motzkus at the University of Heidelberg.
Summary of Scientific Activities
329
Figure 1: Typical pump-probe photoelectron spectrum of acetone acquired
with the magnetic bottle spectrometer.
periods above 10 min, which is still too short to acquire timeresolved photo-electron spectra (1-5 h). During summer 2010
this experiment is being transferred to a laboratory at the
TUM, where our studies will be improved and continued.
The future range of molecules to be investigated will target
complex biomolecules, where we hope to decipher the
intramolecular energy transport via electron/hole (exciton)
mobility. We suspect bond-breaking processes initiated by
photoexcitation to proceed extremely fast in many such
compounds. These experiments will be continued in a joint
effort by R. Kienberger at the TUM and E. Riedle at the
LMU.
Full Resolution of electronic and nuclear Dynamics
in elementary chemical Reactions by Photoelectron
Spectroscopy with 100-as to 10-fs Pulses
Coherent Electron Dynamics and Dephasing in isolated Molecules and molecular Nano-Architectures
In these studies we have concentrated on optically excited
elementary reactions that are so fast that electronic and
nuclear dynamics remain strongly coupled. They address
direct and barrierless processes in molecules that occur after
optical excitation in areas of the potential energy surfaces
that cannot be reached by probe radiation from the ground
state (“dark states”). We use time-resolved photo-electron
spectroscopy that has not yet been implemented with the
intrinsic speed of the electronic and nuclear dynamics,
typically 20 fs for prototypical organic molecules. The
experimental set-up is operational as a magnetic bottle
which has been calibrated with rare gases like argon or xenon
and with diatomics like NO and nitrogen. We have initiated
studies of ultrafast dynamics in molecules like acetone (see
Figure 1), polyfluorobenzenes and diaminobenzonitrile
at UV-wavelengths between 250 nm and 300 nm and in
the visible. We have studied more extensively benzene in a
250 nm pump 208 nm probe experiment, where we expect
very fast transitions according to [8]. The SCF molecular
beam technique [6] has been established and tested with
benzene molecules. At present, this technique can however,
until further improvement, not support data collection
Under this subject we have recently aimed for controlling
electron wave packet motion on molecular length scales and
on time scales where the nuclear coordinates remain frozen.
Such studies hold promise of gaining unprecedented insight
into intra- as well as inter-molecular charge and energy
transport and electronic dephasing in molecules placed in
different environments, with ramifications for molecular
electronics, molecular magnetism, bio-nanotechnology and
bio-informatics. With waveform-controlled few-cycle UV/
VUV pulses available it should be possible to launch an
electron wavepacket at a specific site of a molecule with
sub-fs timing precision, control its subsequent motion with
a tailored field of the excitation pulse and monitor it by subfs photoelectron spectroscopy. Our questions are: How do
the motion and decay of the wavepacket occur in isolated
molecules and how can electrons propagate in molecular
nano-assemblies and devices? How is energy eventually
dissipated and how does dephasing and energy dissipation
depend on the environment? Suitable experiments will
study isolated molecules in the gas phase and subsequently
extend attosecond electron control and spectroscopy to
supramolecular architectures on surfaces.
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Laser Chemistry Emeritus Group
Summary of Scientific Activities
Figure 2: Time resolved pump-probe spectra of D2 two-photon ionization
at photon energies of 38 eV from measurements at FLASH, DESY (see
[10]).
Figure 3: 2D electron-momentum distribution from He strong field
ionization (linear scale) for a CE phase with maximum asymmetry.
The x-axis is the longitudinal momentum, the y-axis is the momentum
transverse to the polarization direction. Structures are due to electron
wave-packet interference (see [11]).
In collaboration with the Institute for Biochemistry in
Münster self-assembled monolayers (Langmuir-Blodgetfilms) of oligopetides have been synthesized. The study
of such monolayers with the means of four-wave-mixing
and photoelectron spectroscopy has been, however, at first
disencouraging. Apart from rapid sample degradation
local charge build-up strongly affected the resolution of the
miniature surface spectrometer that had been constructed for
this purpose. As a next step it is therefore foreseen to switch
to thio-bridged self-assembled monolayers on gold-surfaces,
which supposedly will reduce parasitic surface charge buildup. It is envisioned to produce and characterize such SAM’s
in a close collaboration with the group of J. Barth at the
TUM. In the laboratory of R. Kienberger these SAM’s can
be further investigated by femtosecond and attosecond time
resolved spectroscopy.
observed in several previous experiments. 2009 we have been
able to perform the first XUV-XUV pump-probe experiments
on a molecule [9] (see Figure 2). Target atoms and molecules
have been rare gases, molecular Hydrogene, Oxygene and
Nitrogene, as well as Ethene. The collected experimental
data was so rich that large part of it is still under evaluation.
The dissociation of Hydrogene from highly excited states
has been published recently [10]. Other interesting data on
Oxygene shows pair formation at extended internuclear
distances. The theoretical understanding of these processes
is under development in a collaboration with A. Scrinzi and
M. Nest. High kinetic energy channels have been understood
from a semiclassical model developed by U. Tumm. The
XUV-split-mirror is already scheduled for future experiments
at FLASH and SPRING8 in collaboration with M. Kling
(MPQ) and J. Ullrich at the MPI for Nuclear Physics.
XUV-XUV Time-Resolved Spectroscopy of highly
excited States in diatomic and polyatomic
Molecules
Attosecond Real-Time Observation and Control
of Strong-Field Ionization and Electron-Electron
Rescattering in Atoms and Molecules
Ultrabright XUV laser sources like FLASH at DESY
and SPRING8 in Japan open the opportunity to study
highly excited potential energy surfaces using pump-probe
experiments. The resulting photoelectron and ion kinetic
energy release spectra are acquired and analyzed on a
single event basis, giving access to a unique characterization
of the underlying physics. The spectroscopy of choice
here is reaction microscopy – as invented by the group of
R. Moshammer at the MPI for Nuclear Physics [5]. The
MPQ has contributed to these experiments at FLASH with a
unique split-mirror design of unprecedented capabilities: 3D
nanometer accuracy, extreme stiffness, fully bake-able, and
vacuum compatible up to 10-12 mbar. This advanced splitmirror design finally resolved reproduce-ability problems
Sub-cycle ionization in a time window of few attoseconds
creates extremely short electron bunches which can be
efficiently rescattered onto the parent ion within the
consecutive laser half cycle. This opens the opportunity
to study electron wave-packet interference via recoil ionmomentum spectroscopy. The rescattered electron wavepacket interferes with parts of itself which have been
remaining in the atom. This is also called „double-slits in
time“ (see Figure 3). The resulting photoelectron energy
and angular distribution contains geometrical information
on the bound part of the wave-function. The rescattered
part can be treated, in a first approximation, as a plane
wave. As a consequence, for extremely short laser pulses
the photoelectron energy spectrum deviates from the above
Laser Chemistry Emeritus Group
Summary of Scientific Activities
331
Figure 4: (a) Spatial distribution of Xenon charge states obtained at FLASH recorded at the beam waist. The beam propagation axis is vertical. (b) Blue
circles: plot of the width distribution (FWHM) along the beam propagation axis. Black line: Gaussian optics fit to the experimental data.
threshold multi-photon picture where energies are spaced by
photon energies. Instead, interference leads to a momentum
spacing. The 3D-evaluation of such data has been
associated with the name „electron holography“ [11]. Such
measurements have become accessible from combination of
CEO-phase stable few-cycle lasers with reaction microscopy.
Results on He have been published, and further study of
diatomics has been scheduled for the end of 2010. These
experiments will be continued between M. Kling, and
J. Ullrich and coworkers.
Spatially resolved Multiphoton XUV Ionization
We have recently developed a novel technique to perform
spatially resolved photoionization-yield measurements of
gas phase ions created in the focus of XUV-pulses produced
for example at the FLASH facility at DESY. The advantage
of this technique has been termed ‘ion microscopy’ and
it overcomes the limitations encountered in standard
experiments where the ion yield is usually integrated over
the focal volume and recorded as a function of the peak
intensity in the focus. Our new technique tackles this
problem from two sides, as it allows for intensity resolved ion
yield measurements and at the same time provides a precise
method for non-invasive, in situ focus diagnostics. The ability
to characterize the quality of the focus is a crucial step if
high peak intensities are to be achieved, especially at short
wavelength when the multi-layer mirror technology is close
to its limits. The ion microscope can map the distribution of
ions created in the laser focus and contained in the object
plane onto a position sensitive detector located in the image
plane. The magnification is of the order of 100 and the
resolution is approximately 2 µm (see Figure 4). Gating the
detector with a 7 ns time window enables us to mass select
individual charge states. Assuming the ion distribution to
be symmetric under rotation about the beam propagation
axis, the full 3D distribution can be recovered after Abelinversion of the data. We have applied ion microscopy to the
characterization of an XUV focus obtained using a spherical
multi-layer mirror of 50 cm focal length. FLASH pulses of
13 nm wavelength have been used in the single-bunch mode
to generate xenon ions with charge states up to Xe7+. Images
of the spatial ion distributions of the charge states Xe2+ to
Xe7+ were recorded at various positions z along the beam
propagation axis by translating the focussing mirror along
that direction. The observed beam geometries are closely
Gaussian. Further experiments with the ion microscope are
scheduled by the AFS-group.
Cluster-induced Desorption of Surface Adsorbates
Neutral molecular clusters of a size of 1000 to 10000
molecules bridge the situation of the single molecule in the
gas phase and the properties of a liquid or solid. Such clusters
can be easily brought into the gas phase but they are large
enough to serve as a solvent for atoms, molecules, atomic and
molecular ions, as well as electrons. Furthermore, when such
clusters impact on a solid surface, the kinetic energy of the
clusters has to be redistributed, leading to a strong heating
of the system. Thus, processes which are not accessible
by conventionally heating of the system can be activated.
Depending on the state of charge of the adsorbate and its
surface configuration, both ionic desorption channels as well
as neutral desorption with subsequent charge separation in
the cluster were observed.
In the case of cluster-induced desorption of biomolecules,
the transient matrix which is provided by the cluster during
the cluster-surface collision is of special importance for
the desorption process. It is not only expected to facilitate
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Laser Chemistry Emeritus Group
Summary of Scientific Activities
Figure 6: Satellite line spectra in the Na + CF4 system: (a) Na(3d) signal
in a molecular beam optical collision experiment; (b) secondary Na
fluorescence signal in a thermal gas mixture [13].
Figure 5: Artist’s view of cluster-induced desorption of biomolecules.
During cluster-surface impact, the SO2-cluster both provides the energy
for the desorption process as well as it serves as a transient matrix. This
transient matrix leads to efficient cooling of the system and fragmentation
of the biomolecules is suppressed. As a result of further evaporative
cooling, bare or almost bare molecular ions are detected in the mass
spectrum (as shown in the inset for the desorption of insulin).
desorption by an effective reduction of the desorption
barrier but shattering of the cluster leads to a very fast
cooling of the system within some ten picoseconds. During
that short period of time, no efficient energy transfer into the
vibrational degrees of freedom of the biomolecule is possible
thus leading to a fragmentation-free desorption process.
Subsequent evaporation of further SO2 adducts leads to bare
or almost bare biomolecular ions in the gas phase which can
be investigated by means of mass spectrometry and other
analysis techniques (compare Figure 5 and [7]).
The origin of gas phase ions after cluster-surface collision
has been investigated in detail with the help of alkali halides
applied on various substrates. It could be shown that an
ionic desorption channel is operative as long as the applied
ions are efficiently screened from charge redistribution with
a conductive substrate. For cations, this can be realized by
an insulating surface layer as shown in the case of SiO2. If
the cations are in direct contact with a conductive substrate
surface, charge redistribution leads to a neutral desorption
channel with subsequent charge separation in the cluster
into cation and electron. In the case of the halide anions,
a hydration shell is sufficient to screen the ions from charge
redistribution and enable an ionic desorption channel which
is suppressed when the hydration shell is desorbed.
In summary, cluster-induced desorption enables the
observation of otherwise inaccessible reaction pathways
such as fragmentation free desorption of biomolecules or
ionic desorption of alkali halides from various surfaces. The
experiments will be continued by M. Dürr at the University
for Applied Science in Esslingen.
Laser Collision Spectroscopy and optical Collisions
Spectroscopic studies of collision complexes provide access
to the interactions and the dynamics of a collisional process.
Combined with the molecular beam technique the direct
observation of collision geometries in atom collisions with
rare gases or molecules becomes possible. Nonadiabatic
interactions related to the degeneracy of the excited electronic
states have been investigated and characterize the role of
spin-orbit coupling and of the conical intersections typical
of molecular systems. Atom-atom excited state potentials
have been obtained with spectroscopic accuracy from the
Stueckelberg oscillatory structure from the differential
optical collision cross sections [12].
Recently a satellite structure related to the dipole forbidden
Na(3s-3d) transition was found in a Na/CF4 gas mixture in
the UV region. The direct verification of Na(3d) atoms was
achieved by monitoring them via a transition to a definite
Rydberg state. With this the underlying process is
Na(3s) + CF4(n=1) + hν -> Na(3d) + CF4(n=1)
leading to a line shifted by one vibrational quantum from
the Na(3s-3d) transition (Figure 6). The mechanism is
understood by an interaction of transition dipoles of the
Laser Chemistry Emeritus Group
Summary of Scientific Activities
333
33, 7, 723-25, 2008.
[3] E.A. Torres, K.-L. Kompa, F. Remacle, R.D. Levine,
Chem. Phys. 347, 1-3, 531-45, 2008.
[4] Ackermann W., et al., Nature Phot. 1, 6, 2007.
[5] R. Moshammer, M. Unverzagt, W. Schmitt, J. Ullrich,
H. Schmidt-Böcking, Nucl. Instr. & Methods 108, 4, 425-45,
1996.
[6] W. Christen, S. Grigorenko, K. Rademann, Rev. Sci. Instr.
75, 11, 5048-49, 2004.
[7] C. R. Gebhardt, A. Tomsic, H. Schröder, M. Dürr, K.-L.
Kompa, Angewandte Chemie Int. Edition 48, 23, 4162-65,
2009.
[8] D.S.N. Parker, et al., Chem. Phys. Lett. 469, 43–47, 2009.
[9] Y. H. Jiang, et al., Phys. Rev. A 81, 051402 (R), 2010.
[10] Y. H. Jiang, et al., Phys. Rev. A 81, 021401 (R), 2010.
[11] R. Gopal, et al., Phys. Rev. Lett. 103, 053001, 2009.
[12] Rebentrost, et al., J. Chem. Phys. 128, 224307, 2008.
[13] V.A. Alekseev, et al., J. Chem. Phys. 129, 201102, 2008.
Figure 7: Silicon SHG spectrum originating from surface states and bulklike transitions.
electronic Na(3s-3p) and the vibrational CF4(0-1) modes.
The process presents a nice example for a general class of
laser-induced collisions accompanied by a simultaneous
transfer of electronic and vibrational energy.
Nonlinear optical Response of Semiconductor
Surfaces
Laser surface interactions are a primary source of information
in surface science. Nonlinear optical methods like second
harmonic (SHG) and sum frequency (SFG) generation have
found many applications to semiconductor surfaces because
of their intrinsic surface sensitivity. Of particular interest is
the study of surfaces states arising from dangling bonds on
a semiconductor like silicon. Our theoretical investigations
aim at the mechanisms characterising the nonlinear optical
response of a silicon surface. We use a tight-binding
Hamiltonian for the electronic structure of a cluster or slab
model of a semiconductor surface. The response is calculated
in the density matrix formalism for the one- and two-photon
response. For the cluster model the inclusion of two-particle
excitonic interactions was realized numerically on a highly
parallelized computer architecture. The results provide a first
understanding of the role of excitonic interactions on the
various structures seen in a SHG spectrum originating from
surface states and bulk-like transitions (Figure 7).
References
[1] P. Baum, S. Lochbrunner, E. Riedle, Opt. Lett. 29, 14,
1686-88, 2004.
[2] K. Kosma, S.A. Trushin, W.E. Schmid, W. Fuß, Opt. Lett.
Max-Planck-Institut für Quantenoptik · Progress Report 2009/2010
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Laser Chemistry Emeritus Group
Group Members
Survey
of the Research Activities
Survey of the Research Activities
Laser Chemistry
Project
Objective
Team
Exploring molecular
Computing using Infrared
Spectroscopy
Development of coherent control
schemes towards information
technologies based on molecular
computing.
K.-L. Kompa, E. Torres,
H. Skenderović
External collaborators:
M. Motzkus26, R. de Vivie-Riedle19,
G. Pichler16, F. Remacle14,
R.D. Levine13
Investigation of electronic
and nuclear Dynamics
in elementary chemical
Reactions
Decipher elementary chemical
reactions with strong electronic and
nuclear coupling at timescales below
20 fs.
M. Lezius, K.-L. Kompa,
R. Kienberger
External collaborators:
P. Lang19, E. Riedle19, Ch. Hohmann19,
I. Pugliesi19, K. Karki22
Coherent Electron
Dynamics and Dephasing
in isolated Molecules
and molecular NanoArchitectures
Investigation of adiabatic electron
dynamics and charge transfer in
complex systems.
M. Lezius, K.-L. Kompa,
R. Kienberger
External collaborators:
J. Barth22, P. Feulner22, H.-J. Galla27,
R.D. Levine13, K. Karki22
XUV-XUV time-resolved
Spectroscopy of highly
excited States
Investigate highly excited states in
small molecules with pump-probe
schemes using ultra-intense XUV
sources.
M. Lezius, O. Herrwerth, M.F. Kling
External collaborators:
Y.H. Jiang3, A. Rudenko4, E. Plesiat5,
L. Foucar3, M. Kurka3, K.U. Kühnel3,
Th. Ergler3, J.F. Perez-Torres5,
F. Martin5, J. Titze6, T. Jahnke6,
R. Dörner6, J.L. Sanz-Vicario7,
M. Schöffler8, J. van Tilborg8,
M. Schulz23, A. Belkacem8, K. Ueda9,
T.J.M. Zouros10, S. Düsterer11,
R. Treusch11, C.D. Schröter3,
R. Moshammer3, J. Ullrich3,4,
A. Scrinzi21, M. Nest20
Attosecond Observation
and Control of strong-field
Ionization and Electron
Rescattering in Molecules
Electron rescattering in strong fields
opens the opportunity to investigate
wavepacket dynamics in atoms and
molecules on attosecond time-scales.
M. Lezius, K.-L. Kompa, M. Kling,
B. Bergues, M. Schultze,
O. Herrwerth, E. Goulielmakis,
M. Uiberacker
External collaborators:
N. Johnson25, R. Gopal3, J. Ullrich3,4,
R. Moshammer3, G.G. Paulus24,
A.M. Sayler24, T. Ratje24 ,
A. Senftleben3 , K. Simeonidis3,
Th. Ergler3, M. Dürr3, M. Kurka3,
K.-U. Kuhnel3, S. Tschuch3,
C.-D. Schröter3, A. Rudenko4 ,
Th. Uphues2, I. Ben-Itzhak25
Laser Chemistry Emeritus Group
Summary
Survey
of the
of Scientific
ResearchActivities
Activities
335
Laser Chemistry (cont.)
Project
Objective
Team
Spatially resolved
Multiphoton Ionization
Application of the ion microscopy
technique to intense field ionization
in the wavelength range from 10 to
1000 nm.
H. Schröder, M. Lezius, K.-L. Kompa,
B. Bergues, M. Schultze, M.F. Kling,
A. Wirth ,W. Helml, O. Herrwerth,
M. Hofstetter , G. Marcus,
R. Kienberger, F. Krausz
External collaborators:
A. Rudenko4 , K.-U. Kühnel3,
C.D. Schröter3, R. Moshammer3,
J. Ullrich3,4, R. Treusch11,
S. Düsterer11, P. Lang19
Laser Collision
Spectroscopy and optical
Collisions
Spectroscopic studies of collision
complexes to understand dynamics
and interactions of collisions.
F. Rebentrost
External collaborators:
V.A. Alekseev28, J.O. Grosser28,
O. Hoffmann28, C. Figl28,
R. Goldstein28, D. Spelsberg28
Nonlinear optical Response
of Semiconductor Surfaces
Theoretical investigations that aim
at the mechanisms characterising the
nonlinear optical response of a silicon
surface.
F. Rebentrost
External collaborators:
M. Stamova
Cluster induced Desorption
of Surface Adsorbates
Soft desorption of biomolecules.
H. Schröder, K.-L. Kompa,
External collaborators:
Ch. Gebhardt, M. Dürr29
1. Group of Prof. Krausz at MPQ
15. University of Heidelberg
2. Group of Prof. Kling at MPQ
16. University of Zagreb,
3. Max-Planck-Institut für Kernphysik, D-69117 Heidelberg, Germany
17. Indian Institute of Technology
4. Max-Planck Advanced Study Group at CFEL, D-22607 Hamburg, Germany
18. Universite de Laval, Quebec
5. Departamento de Quimica, Universidad Autonoma de Madrid, E-28049 Madrid,
19. Institute for Biomolecular Optics, Ludwig-Maximilians-Universität, München
Spain
20. Department for Chemistry, Technical University Munich
6. Institut für Kernphysik, Universität Frankfurt, D-60486 Frankfurt, Germany
21. Arnold Sommerfeld Center, Ludwig-Maximilians University Munich
7. Instituto de Fisica, Universidad de Antioquia, Medellin, Colombia
22. Experimental Physics E11, Technical University Munich
8. Lawrence Berkeley National Laboratory, Berkeley, California 94720, USA
23. Missouri University of Science & Technology Rolla, Missouri 65409, USA
9. Institute of Multidisciplinary Research for Advanced Materials, Tohoku University,
24. Institut für Optik und Quantenelektronik, Friedrich-Schiller-Universität, Jena, 07743
980-8577 Sendai, Japan
Germany
10. Department of Physics, University of Crete, Post Office Box 2208, GR-71003
25. Kansas State University, Manhattan , KS 66506
Heraklion, Crete, Greece
26. Inst. For Physical Chemistry, Heidelberg University, Germany
11. DESY, D-22607 Hamburg, Germany
27. Inst. f. Biochemistry, Wilhelms-University Münster, Gemany
12. Weizmann Institute of Science , Rehovot
28. Institute for Gravitational Physics, Leibniz University Hannover, Germany
13. The Hebrew University, Jerusalem, Israel
29. Universität Esslingen, Germany
14. Université de Liège, Belgium
Max-Planck-Institut für Quantenoptik · Progress Report 2009/2010
336
Laser Chemistry Emeritus Group
Selected Reprints
Selected Reprints
1) Matrix-free formation of gas-phase biomolecular ions
by soft cluster-induced desorption
C.R. Gebhardt, A.Tomsic, H. Schröder, M. Dürr,
K.-L. Kompa
Angewandte Chemie International Edition 2009 48,
4162 –4165 (2009)
MPQ Progress Report: page 337
2) Simultaneous optical excitation of Na electronic and
CF4 vibrational modes in Na+CF4 collisions
V.A. Alekseev, J. Grosser, O. Hoffmann, F. Rebentrost
The Journal of Chemical Physics 129, 201102 (2008)
MPQ Progress Report: page 338
3) Excitonic effects in the nonlinear optical response of a
Si(111) surface
M. Stamova, F. Rebentrost
Physica Status Solidi B 247, 8, 2012–2016 (2010)
MPQ Progress Report: page 339
4) Cyclohexadiene ring opening observed with 13 fs resolution: coherent oscillations confirm the reaction path
K. Kosma, S.A. Trushin, W. Fuß, W.E. Schmid
Physical Chemistry Chemical Physics 11, 172–181 (2009)
MPQ Progress Report: page 340
5) Three-dimensional momentum imaging of electron
wave packet interference in few-cycle laser pulses
R. Gopal, K. Simeonidis, R. Moshammer, Th. Ergler,
M. Dürr, M. Kurka, K.-U. Kühnel, S. Tschuch,
C.-D. Schröter, D. Bauer, J. Ullrich, A. Rudenko,
O. Herrwerth, Th. Uphues, M. Schultze, E. Goulielmakis,
M. Uiberacker, M. Lezius, M. F. Kling
Physical Review Letters 103, 053001 (2009)
MPQ Progress Report: page 341
6) Tracing direct and sequential two-photon double
ionization of D2 in femtosecond extreme-ultraviolet laser
pulses
Y.H. Jiang, A. Rudenko, E. Plésiat, L. Foucar, M. Kurka,
K.U. Kühnel, Th. Ergler, J.F. Pérez-Torres, F. Martín,
O. Herrwerth, M. Lezius, M.F. Kling, J. Titze, T. Jahnke,
R. Dörner, J.L. Sanz-Vicario, M. Schöffler, J. van Tilborg,
A. Belkacem, K. Ueda, T.J.M. Zouros, S. Düsterer,
R. Treusch, C.D. Schröter, R. Moshammer, J. Ullrich
Physical Review A 81, 021401 (2010)
MPQ Progress Report: page 342
7) Investigating two-photon double ionization of D2 by
XUV-pump–XUV-probe experiments
Y.H. Jiang, A. Rudenko, J.F. Pérez-Torres, O. Herrwerth,
L. Foucar, M. Kurka, K.U. Kühnel, M. Toppin, E. Plésiat,
F. Morales, F. Martín, M. Lezius, M. F. Kling, T. Jahnke,
R. Dörner, J.L. Sanz-Vicario, J. van Tilborg, A. Belkacem,
M. Schulz, K. Ueda, T.J.M. Zouros, S. Düsterer, R. Treusch,
C.D. Schröter, R. Moshammer, J. Ullrich
Physical Review A 81, 051402 (2010)
MPQ Progress Report: page 343
Laser
Chemistry
Emeritus
Group
Quantum
Dynamics
Division
Selected
Chem. Int. Ed. 2009, 48, 4162-4165
SummaryReprints
of ScientificAngew.
Activities
337
Communications
DOI: 10.1002/anie.200804431
Mass Spectrometry
Matrix-Free Formation of Gas-Phase Biomolecular Ions by Soft
Cluster-Induced Desorption**
Christoph. R. Gebhardt,* Anna Tomsic, Hartmut Schr�der, Michael D�rr,* and Karl L. Kompa
Mass spectrometry of biological macromolecules has developed into a key technology for fast routine analysis in
biotechnology.[1] A critical issue is the efficient transfer of
nonvolatile biomolecules out of their sample solution into the
gas phase in combination with their concomitant ionization.
Established standard methods are matrix-assisted laser
desorption and ionization (MALDI)[2] and electrospray
ionization (ESI).[3] MALDI comprises laser desorption of
analyte molecules that have been embedded in a matrix that is
strongly absorbing at the laser wavelength;[2] in ESI, the
sample solution is directly dispersed into charged nanoscopic
droplets by a combination of gas injection and a strong
electrostatic field applied to a microcapillary.[3] Alternative
methods include massive cluster impact ionization (MCI),[4]
secondary ion mass spectrometry (SIMS),[5] and electrospray
droplet impact (EDI) in combination with SIMS,[6] which all
make use of an impacting charged particle to desorb and
ionize the biomolecules. Using a spray of charged droplets,
desorption electrospray ionization (DESI)[7] combines the
ESI scheme with a soft desorption process[8] and allows for
mass spectrometry of biomolecules under ambient conditions.
Herein we show that neutral molecular clusters of 103 to
104 SO2 molecules can also be used for the desorption and
ionization of biomolecules. Cluster impact on arbitrary
surfaces pretreated with biomolecules efficiently creates
cold, desolvated, gas-phase biomolecular ions as large as
6000 u (1 u = 1 unified atomic mass unit) without any need for
preparation of the biomolecules in a special matrix or means
of postionization after desorption. Since the cluster provides
not only the energy for the desorption process but also a
transient matrix during the process, the biomolecules were
found to be desorbed without any fragmentation. The time[*] Dr. C. R. Gebhardt, Dr. A. Tomsic, Dr. H. Schr�der,
Prof. Dr. K. L. Kompa
Max-Planck-Institut f�r Quantenoptik
Hans-Kopfermann-Strasse 1, 85748 Garching (Germany)
Fax: (+ 49) 89-3290-5313
E-mail: christoph.gebhardt@bdal.de
Prof. Dr. M. D�rr
Hochschule Esslingen
Fakult�t Angewandte Naturwissenschaften – Chemieingenieurwesen
Kanalstrasse 33, 73728 Esslingen (Germany)
Fax: (+ 49) 711-397-3502
E-mail: michael.duerr@hs-esslingen.de
[**] This work was supported by the German government through
BMBF under the EEF program. The authors would like to thank W.
Ritt for substantial experimental help.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.200804431.
4162
scale on which that energy is redistributed after cluster–
surface collision and biomolecule pickup is shown to be a key
to the understanding of the soft desorption mechanism.
Furthermore, the state of charge of the desorbed molecular
ions in the gas phase can be controlled by the pH value of the
original sample solution.
The experiment is sketched in Figure 1.[9, 10] The SO2
clusters are seeded in a He beam and hit the collision target
under vacuum. The biomolecules have been deposited on the
Figure 1. Upper left: Schematic depiction of the cluster-impact experiment. After the neutral cluster beam hits the sample surface, charged
fragments carrying biomolecules are extracted by the biased grid (G).
Mass analysis is performed in the TOF mass spectrometer oriented
perpendicular to the primary beam. Skimmer (S) and aperture (A)
allow for beam collimation. Lower right: schematic depictions of
cluster impact and subsequent desorption of biomolecules from a
surface (not to scale).
target by simply drop-casting the respective solution. Upon
impact of the neutral cluster beam on the sample surface, the
abundant formation of free molecular ions is detected with a
pulsed time-of-flight (TOF) mass spectrometer, which is
oriented perpendicular to the beam axis. As an example,
Figure 2 shows the cationic mass spectrum from a TiN surface
pretreated with a mixed oligopeptide solution. The amount of
substance of each of the constituents was 1010 mol. As all the
spectra, it was baseline-corrected and adjusted for the massdependent efficiency of the microchannel plate detector.[11]
The major peaks are easily assigned to the singly charged,
bare oligopeptides of the original solution. Furthermore, we
observe doubly charged oligopeptide ions, oligopeptide ions
with SO2 adducts, and dimers of oligopeptides. Whereas most
experiments where performed with a total amount of analyte
� 2009 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2009, 48, 4162 –4165
Max-Planck-Institut für Quantenoptik · Progress Report 2009/2010
338
Quantum
Laser
Chemistry
Dynamics
Emeritus
Division
Group
Group
Members
Selected
Reprints
The Journ. of Chem. Phys. 129, 201102 (2008)
THE JOURNAL OF CHEMICAL PHYSICS 129, 201102 2008
Simultaneous optical excitation of Na electronic and CF4 vibrational modes
in Na+ CF4 collisions
V. A. Alekseev,1 J. Grosser,2,a O. Hoffmann,2 and F. Rebentrost3
1
Department of Physics, St. Petersburg State University, Peterhof 198504, Russia
Institut für Gravitationsphysik, Leibniz Universität Hannover, 30167 Hannover, Germany
Max-Planck-Institut für Quantenoptik, 85748 Garching, Germany
2
3
Received 30 September 2008; accepted 30 October 2008; published online 24 November 2008
We report on the ultraviolet excitation of Na3s + CF4 collision pairs in a crossed molecular beam
experiment. We observe Na3d collision products originating from the process Na3s + CF4
3 = 0 + h → Na3d + CF43 = 1. The spectral intensity distribution of the collision products and
the prevailing small angle scattering confirm a previously proposed long range dipole-dipole
mechanism. We report velocity-resolved spectra and a comparison to preliminary numerical results
based on collisional broadening theory. Polarization experiments suggest future potential for the
observation of collision geometries. © 2008 American Institute of Physics.
DOI: 10.1063/1.3028653
The nonresonant optical excitation of atoms during thermal energy collisions has been studied intensely in crossedbeam experiments.1 It provides information on potential
curves, nonadiabatic transitions, and geometrical properties
of collision pairs. Also the controlled manipulation of differential cross sections by polarized light has been
demonstrated.2 In all cases, the second collision partner was
effectively inert, i.e., its electronic and vibrational states remained unchanged. We deal here with the process
Na3s + CF43 = 0 + h → Na3d + CF43 = 1,
where the dipole forbidden electronic transition to Na3d
becomes possible by the simultaneous excitation of the vibrational 3 mode in CF4. The process was recently observed
in a Na+ CF4 gas cell experiment3 and has now been confirmed and investigated in more detail using a crossed-beam
setup.
The experimental setup1 shown in Fig. 1 consists of the
Na and CF4 beams and of two pulsed light beams, which all
intersect in a scattering volume. A rotatable differential detector is used to monitor the electronically excited Na atoms.
A UV light beam with wavelength near 328 nm is generated
by frequency doubling the output of a dye laser. It serves for
the optical excitation during the Na+ CF4 collision. A second
dye laser operating near 822 nm is part of the detection
scheme. Na3d atoms normally cascade to the 3s state in a
short time, prohibiting a state selective detection at a distant
detector. The 822 nm photons therefore transfer Na3d atoms to a long lived Rydberg state. Rydberg atoms arriving at
the detector are field ionized and counted. With all four
beams in operation, a scan of the detector output versus the
detection laser wavelength yields a series of lines, which are
easily identified as 3d → nf transitions,4 see the inset in Fig.
1. All subsequent results were obtained with the detection
laser operating on the 3d → 32f line. The detector output is
a
Electronic mail: jgrosser@ceres.amp.uni-hannover.de.
0021-9606/2008/12920/201102/4/$23.00
proportional to the Na3d population after the collision. The
Na atoms in the beam have a broad thermal velocity distribution, leading to a broad distribution also after the collision.
With the laser pulses as the trigger signal, we measure the
velocity v of every detected Na atom by its time of flight to
the detector. The procedure replaces a velocity selection before the collision; under the present conditions, the velocities
before and after the collision are, however, practically identical.
Figure 2a shows the measured Na3d intensity as a
function of the UV wavelength . A background signal of
unknown origin of typically 0.01 count per laser pulse,
which appears already when the CF4 beam is switched off, is
subtracted from all experimental results. The scale at the top
of Fig. 2a indicates the detuning with respect to the
Na3s → 3d transition at 0 = 342.8 nm. The signal peak
closely matches the energy of the 3 vibrational mode of CF4
at 1280 cm−1.5 Figure 2b shows the result of a recent Na
+ CF4 gas cell experiment,3 where Na3d excitation was detected indirectly by secondary fluorescence. The two results
n = 33 32 31 30
821
822
29
CF beam
Rydberg detector
823 nm
detection laser
32
3
excitation laser 328 nm
Na beam
FIG. 1. The setup of the crossed-beam experiment. The inset shows a spectral scan of the detection laser identifying the excited Na3d atoms. The
lines at the top mark the 3d → nf line positions from Ref. 4; the 3d finestructure splitting is not resolved.
129, 201102-1
© 2008 American Institute of Physics
Downloaded 21 Jul 2010 to 130.183.91.79. Redistribution subject to AIP license or copyright; see http://jcp.aip.org/jcp/copyright.jsp
Summary
Selected Reprints
of ScientificPhys.
Activities
Status Solidi B 247, 8, 2012–2016 (2010) 339
Excitonic effects in the nonlinear
optical response of a Si(111) surface
solidi
pss
status
Phys. Status Solidi B 247, No. 8, 2012–2016 (2010) / DOI 10.1002/pssb.200983949
physica
Quantum
Laser
Chemistry
Dynamics
Emeritus
Division
Group
b
www.pss-b.com
basic solid state physics
Maria Stamova** and Frank Rebentrost*
Max-Planck-Institut für Quantenoptik, 85748 Garching, Germany
Received 1 November 2009, revised 3 March 2010, accepted 11 March 2010
Published online 22 June 2010
Keywords nonlinear optics, optical response functions, silicon, surfaces and interfaces
* Corresponding
** e-mail
author: e-mail far@mpq.mpg.de, Phone: þ49-89-3205713, Fax: þ49-89-3205200
mos@mpq.mpg.de
We discuss methods to calculate the linear and nonlinear optical
spectra for cyclic cluster models of an ideal Si(111) surface.
The cluster approach offers the possibility to implement the
excitonic effects due to the Coulomb interaction between
electron and hole in a relatively straight-forward way. In order
to appproximate a situation resembling a surface we use
clusters with several hundreds of Si atoms. The electronic
structure is obtained from a tight-binding parametrization of the
hamiltonian. A time-dependent density operator formalism is
used to calculate the response functions SðtÞ and Sðt1 ; t 2 Þ for
the optical polarization, which also directly describe the
response to ultrashort pulses. Their Fourier transforms are the
frequency-dependent optical susceptibilities xð1Þ ð�v; vÞ and
xð2Þ ð�v1 � v2 ; v1 ; v2 Þ for second-harmonic (v1 ¼ v2 ) or
sum-frequency generation from surfaces. The excitonic Coulomb interaction is treated in the time-dependent Hartree–Fock
approximation, leading to large sets of differential equations
that are integrated explicitly. The results on the linear
susceptibility are in accord with earlier findings on the excitonic
origin of the relative intensities of the E1 and E2 peaks near 3.4
and 4.3 eV. We present new results on excitonic effects in the
nonlinear spectra and investigate in particular the surfacerelated peaks near 2�
hv ¼ 1.3–1.5 and 2.4 eV that govern the
strong enhancement observed in SHG of clean silicon surfaces.
2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
1 Introduction A theoretical investigation of optical
properties of semiconductor solids and surfaces is nowadays
often performed by density-functional theory (DFT) with
quasiparticle level shifts and including excitonic effects in
the framework of Bethe–Salpeter equation or time-dependent Hartree–Fock (TDHF) [1]. The understanding of the
surface-specific second-order response is however, still
incomplete on a microscopic level. Experimental methods
based on the linear or nonlinear response of surfaces, such
as reflectance anisotropy spectroscopy(RAS) and differential reflectivity spectroscopy(DRS), or sum frequency
generation(SFG) and particularly second harmonic generation, SHG, are characterized by a high surface sensitivity
and capable to cover a wide spectral range. The advantages
are their non-destructive character, insensivity to transparent
ambients, high spectral and temporal resolution for structural
and electronic surface characterisation. The use of ultrashort-pulse lasers in nonlinear spectroscopy methods such as
time-resolved two-photon photoemission and time-resolved
SHG has further advanced the study of the dynamics of
electronic excitations and chemical reactions at surfaces.
In recent years, the nonlinear optical response from clean
and adsorbate covered Si(111) surface has been theoretically
studied in semiempirical and more advanced frameworks,
and the qualitative behaviour of SHG surface spectra
for different surfaces has been successfully described.
Calculations of the surface SHG response, including the
first theoretical description of xð2Þ ð�2v; v; vÞ spectra, have
been performed by semiempirical tight-binding approach for
clean reconstructed and H and As covered Si(111) surfaces
[2–7]. In these calculations, the linear and the nonlinear
optical susceptibilities are evaluated in the frequency
domain, within the independent particle approximation
neglecting many-body and local-field effects. Calculations
based on DFT have also been carried out for the SHG
response of clean and adsorbate covered Si(100) [8, 9] and
Si(111) [10, 11] surfaces.
Calculations of optical spectra for surfaces are unavoidably more complicated and computationally more demanding than those for the crystal bulk. Apart from the successful
first-principles determination of linear optical spectra of
some simple Si surfaces [12–14], including excitonic and
2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
Max-Planck-Institut für Quantenoptik · Progress Report 2009/2010
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Phys. Chem. Chem. Phys., 11, 172–181 (2009)
www.rsc.org/pccp | Physical Chemistry Chemical Physics
Cyclohexadiene ring opening observed with 13 fs resolution: coherent
oscillations confirm the reaction path
K. Kosma, S. A. Trushin, W. Fuß* and W. E. Schmid
Received 14th August 2008, Accepted 13th October 2008
First published as an Advance Article on the web 6th November 2008
DOI: 10.1039/b814201g
The third harmonic (270 nm, 11 fs), produced in a short argon cell from Ti-sapphire laser pulses
(810 nm, 12 fs), was used to excite 1,3-cyclohexadiene to its lowest pp* state (1B). Probing was
done by transient ionization by the 810 nm pulses, measuring the yields of the parent and a
fragment ion. As previously found with 10 times longer pulses, the molecule leaves in two steps
(time constants t1, t2) from the spectroscopic (1B) to a dark (2A) state and from there (within t3)
to the ground-state surface. In addition to slightly improved values for t1–t3, we found in all
three locations (L1–L3) on the potentials coherent oscillations, which can be assigned to
vibrations. They are stimulated by slopes (driving forces) of the potentials, and the vibrational
coordinates indicate the slope directions. From them we can infer the path following the initial
excitation: the molecule is first not only accelerated towards CC stretching in the p system but
also along a symmetric CQC twist. The latter motion—after some excursion—also erects and
stretches the CH2–CH2 bond, so that Woodward–Hoffmann interactions are activated after this
delay (in L2). On leaving L2 (the 1B minimum) around the lower cone of the 1B/2A conical
intersection, the wave packet is rapidly accelerated along an antisymmetric coordinate, which
breaks the C2 symmetry of the molecule and eventually leads in a ballistic path to (and through)
the last (2A/1A) conical intersection. The ring opening begins already on the 1B surface; near the
2A minimum it is already far advanced, but is only completed on the ground-state surface.
1. Introduction
Photochemical ring opening of 1,3-cyclohexadiene (CHD) to
Z-hexatriene (HT) and its reverse is a prototype of pericyclic
reactions.1 The stereochemistry of such reactions with steroid
derivatives has played an important role in the derivation of
the Woodward–Hoffmann (WH) rules.2,3 Cyclohexadiene/
hexatriene interconversion is also the basis of many photochromic dyes.4,5 The reaction was therefore much investigated
over the last two decades by quantum-chemical calculations,6–21 by resonance-Raman spectroscopy,22,23 in part with
time resolution,24–28 by transient absorption in solution,29–31
in the gas phase by transient ionization (time-resolved mass
spectroscopy),11,32–34 transient photoelectron spectroscopy35
and time-resolved electron diffraction.36–39 It may be the
system, for which most details are now known on the reaction
path over the different surfaces (see, in particular, ref. 11). This
is probably also the reason, why the system has been studied
for application of shaped pulses for controlling the reaction,
both theoretically12,15,40,41 and experimentally.42,43
As is typical for photoinduced pericyclic reactions1,44 (see
Fig. 3 and 6 below), the molecule is first excited to a ‘‘spectroscopic’’ state (1B2 in C2v, 1B in C2), from where it is
initially accelerated along Franck–Condon active coordinates
(stretches and twists of the p system11). Then also the
CH2–CH2 s bond is stretched and the WH rules are
Max-Planck-Institut für Quantenoptik, D-85741 Garching, Germany.
E-mail: w.fuss(mpq.mpg.de; Fax: +49-89-32905-200
172 | Phys. Chem. Chem. Phys., 2009, 11, 172–181
‘‘turned on’’.11 From there the wave packet falls into a dark
(2A1 or 2A) state; in doing so, it circumvents the 1B/2A
conical intersection (CI) along an antisymmetric (b2 or b,
C2-symmetry breaking) coordinate, as previously suggested.11
Under C2-constraint, the 2A potential has a ‘‘pericyclic
minimum’’ half-way between the reactant and product. From
there, the same b2 distortion slightly lowers the 2A energy and
further leads to a CI, the minimum of the 2A/1A intersection
space.11 From this CI the path branches to the product HT
and the reactant CHD. On the 1B surface the wave packet
travels E55 fs (a time consisting of two phases11,33,34), and its
departure from 2A takes E80 fs.11,33–35 The early work24–28
and electron diffraction36–39 did not have sufficient time
resolution and only detected the ground-state products or
those resulting from reactions in the hot ground state.38,39
The investigations in solution29–31 found the correct total time
(E200 fs) for the process, but could not monitor the
individual steps.
Also in our previous transient-ionization work11,33,34 and
the time-resolved photoelectron spectroscopy35 deconvolution
was necessary, because in particular the UV pump pulses had
durations of 130–150 fs. Recently we developed a simple
source of UV pulses with duration r10 fs45 and demonstrated
its use for time-resolved spectroscopy of metal carbonyl
dissociation.46 It seemed suggestive to measure also the times
of the different phases of CHD ring opening directly. Even
more interesting is the possibility to resolve coherent oscillations. They cannot be revealed by deconvolution. We previously demonstrated, how much information of the reaction
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Rev. Lett. 103, 053001 (2009)
week ending
31 JULY 2009
PHYSICAL REVIEW LETTERS
PRL 103, 053001 (2009)
341
Three-Dimensional Momentum Imaging of Electron Wave Packet Interference
in Few-Cycle Laser Pulses
R. Gopal, K. Simeonidis, R. Moshammer, Th. Ergler, M. Dürr, M. Kurka, K.-U. Kühnel, S. Tschuch, C.-D. Schröter,
D. Bauer, and J. Ullrich
Max-Planck-Institut für Kernphysik, Saupfercheckweg 1, D-69117 Heidelberg, Germany
A. Rudenko
Max-Planck Advanced Study Group at CFEL, D-22607 Hamburg, Germany
O. Herrwerth, Th. Uphues, M. Schultze, E. Goulielmakis, M. Uiberacker, M. Lezius, and M. F. Kling
Max-Planck-Institut für Quantenoptik, Hans-Kopfermann-Strasse 1, D-85748 Garching, Germany
(Received 10 February 2009; published 27 July 2009)
Using a reaction microscope, three-dimensional (3D) electron (and ion) momentum (P) spectra have
been recorded for carrier-envelope-phase (CEP) stabilized few-cycle (�5 fs), intense (�4 �
1014 W=cm2 ) laser pulses (740 nm) impinging on He. Preferential emission of low-energy electrons
(Ee < 15 eV) to either hemisphere is observed as a function of the CEP. Clear interference patterns
emerge in P space at CEPs with maximum asymmetry, interpreted as attosecond interferences of
rescattered and directly emitted electron wave packets by means of a simple model.
DOI: 10.1103/PhysRevLett.103.053001
PACS numbers: 32.80.Rm
Ionization of rare gas atoms such as helium at laser
intensities in the range of 1014 –1015 W=cm2 is usually
described by tunneling, where the valence electron tunnels
through the field suppressed barrier. The electron then
undergoes an oscillatory motion in the electric field to
achieve a final momentum, also known as the drift momentum. In few-cycle pulses (e.g., 5 fs at 800 nm with a
single cycle time period of 2.7 fs), a highly nonlinear
process such as tunneling is confined to a single cycle
around the maximum of the pulse envelope, as illustrated
in Fig. 1. Here, for a sinelike waveform (�CEP ¼ ��=2,
CEP: carrier-envelope-phase, defined as the phase difference between the envelope maximum and the nearest
electric field maximum), two tunneling phases, symmetrically spaced around the zero crossing of the field will lead
to the same momentum, and the corresponding trajectories
will interfere. As first outlined in [1] and detailed in [2], the
wave packet EWP1 that was launched at t1 recollides with
the ionic core thereby being modified in phase and wavefront direction (often dubbed ‘‘Coulomb focusing’’) and
overlaps with an unaffected ‘‘reference’’ wave EWP2 (of
the same electron) launched at t2 . A 3D momentum image
of these electron wave-packet (EWP) interferences therefore represents a time-dependent hologram of the modulations imposed onto EWP1 . Here the attosecond dynamics
of the electron cloud bound to the ion might be imprinted
and can potentially be reconstructed for atomic, molecular,
and cluster targets. In experiments the interaction of the
returning EWP1 with the parent ion core has been used to
retrieve atomic or molecular structure and dynamics
through high harmonic generation, as for, e.g., in [3,4] or
through electron diffraction as in [5]. More recently inter0031-9007=09=103(5)=053001(4)
ferences with EWPs generated by attosecond pulse trains
and steered by an infrared laser pulse [6] have been demonstrated to image the coherent scattering of electrons
from the parent ion [7].
In this Letter we report on the first demonstration of
subcycle EWP interferences seen in high-resolution, 3D
low-energy electron (ion) momentum distributions for
single ionization of He by CEP-stabilized few-cycle
pulses, investigated by a ‘‘reaction microscope’’ [8].
Momentum distributions along the laser polarization axis
(pk ), not only show a CEP-dependent preferential emission
to either hemisphere, but also a corresponding asymmetric
occurrence of interference peaks. The spacing
pffiffiffiffiffiffiffiffiffi between the
peaks, significantly smaller than �pk / 1@! as observed
for longer pulses, is compared with a simple strong field
FIG. 1 (color). Electron trajectories in an ultrashort pulse, with
�CEP ¼ �=2. Electrons born at times ts (s ¼ 1, 2) end up with
the same momentum pðts Þ following the trajectories
R given by the
red and blue curves, respectively: pðts Þ ¼ � 1
ts EðtÞdt, in
atomic units. The right-hand axis is the displacement of the
electron from z ¼ 0.
053001-1
2009 The American Physical Society
Max-Planck-Institut für Quantenoptik · Progress Report 2009/2010
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Phys. Rev. A 81, 021401(R) (2010)
RAPID COMMUNICATIONS
PHYSICAL REVIEW A 81, 021401(R) (2010)
Tracing direct and sequential two-photon double ionization of D2 in femtosecond
extreme-ultraviolet laser pulses
Y. H. Jiang,1 A. Rudenko,2 E. Plésiat,3 L. Foucar,2 M. Kurka,1 K. U. Kühnel,1 Th. Ergler,1 J. F. Pérez-Torres,3 F. Martı́n,3
O. Herrwerth,4 M. Lezius,4 M. F. Kling,4 J. Titze,5 T. Jahnke,5 R. Dörner,5 J. L. Sanz-Vicario,6 M. Schöffler,7 J. van Tilborg,7
A. Belkacem,7 K. Ueda,8 T. J. M. Zouros,9 S. Düsterer,10 R. Treusch,10 C. D. Schröter,1 R. Moshammer,1 and J. Ullrich1,2
1
Max-Planck-Institut für Kernphysik, D-69117 Heidelberg, Germany
Max-Planck Advanced Study Group at CFEL, D-22607 Hamburg, Germany
3
Departamento de Quı́mica C-9, Universidad Autónoma de Madrid, E-28049 Madrid, Spain
4
Max-Planck-Institut für Quantenoptik, D-85748 Garching, Germany
5
Institut für Kernphysik, Universität Frankfurt, D-60486 Frankfurt, Germany
6
Instituto de Fı́sica, Universidad de Antioquia, Medellı́n, Colombia
7
Lawrence Berkeley National Laboratory, Berkeley, California 94720, USA
8
Institute of Multidisciplinary Research for Advanced Materials, Tohoku University, 980-8577 Sendai, Japan
9
Department of Physics, University of Crete, Post Office Box 2208, GR-71003 Heraklion, Crete, Greece
10
DESY, D-22607 Hamburg, Germany
(Received 12 August 2009; published 23 February 2010)
2
Two-photon double ionization (TPDI) of D2 is studied for 38-eV photons at the Free Electron Laser in Hamburg
(FLASH). Based on model calculations, instantaneous and sequential absorption pathways are identified as
separated peaks in the measured D+ + D+ fragment kinetic energy release (KER) spectra. The instantaneous
process appears at high KER, corresponding to ionization at the molecule’s equilibrium distance, in contrast to
sequential ionization mainly leading to low-KER contributions. Measured fragment angular distributions are in
good agreement with theory.
DOI: 10.1103/PhysRevA.81.021401
PACS number(s): 33.80.−b
Two-photon double ionization (TPDI), that is, the interaction of two photons with two electrons, is among the
most fundamental nonlinear processes in atomic [1–8] and
molecular [9–12] physics. It is, thus, considered a benchmark
reaction to advance nonlinear theories and to explore electronelectron correlations in atoms as well as the coupling between
electronic and nuclear motion in molecules beyond the BornOppenheimer approximation (BOA). Sparked by experiments
that have become feasible at intense high harmonics (see, e.g.,
[5,6,9]) or ultrabright, free electron laser (FEL) sources such as
the Free Electron Laser in Hamburg (FLASH) (see, e.g., [7,8])
and puzzled by the intriguing challenges in predicting the
removal of two electrons from He, theoretical interest has
just exploded (see. e.g., [1–4] and references therein). Initial
attempts to calculate TPDI of molecules [11,12] have been
published very recently, which, because of the extreme computational demands, are based on the fixed-nuclei approximation.
As schematically illustrated in Fig. 1 for the D2 molecule,
two different basic TPDI pathways have been discussed in
the literature. For “sequential ionization” (SI) the photons
(purple arrows in Fig. 1) are assumed to be absorbed via
an intermediate stationary state of the ion (assumed to be
the 1sσg ground state of D2 + ) in two steps, which might be
traced in time as indicated by the horizontal arrow. In the
direct or “nonsequential ionization” (NSI) channel instead,
both photons are absorbed simultaneously through a virtual
intermediate state as indicated by the left vertical arrows
in Fig. 1. Despite tremendous theoretical efforts, questions
on the direct ionization of the two He electrons by two
photons are not yet settled, not even on the level of total cross
sections [4].
On the experimental side as well, the accuracy of total cross
section measurements for TPDI has been questioned, since
1050-2947/2010/81(2)/021401(4)
021401-1
either the statistical significance was weak or, for the FLASH
measurements, the intensity of the VUV pulse is not well
known due to the uncontrolled time structure of the pulses,
emerging from the noise in the self-amplified spontaneous
emission (SASE) process. Moreover, in cases where both,
SI and NSI are energetically allowed, it was not possible to
trace both reactions. For two-photon ionization of H2 /D2 there
exists, to the best of our knowledge, only one experiment [9].
It was found that the production of energetic protons and
deuterons proceeds dominantly through two-photon abovethreshold ionization but the available intensity was too low
(3 × 1012 W/cm2 , 42 eV) for TPDI to be observed.
In this Rapid Communication we demonstrate a method to
identify SI and NSI contributions for TPDI of D2 at 38 eV using
a reaction microscope (REMI) [13] to measure the complete
fragment-ion momenta. Exploiting the molecule’s internal
nuclear dynamics (i.e., launching a nuclear wave packet in D2 +
by the absorption of the first photon; see Fig. 1), we are able
to trace SI and NSI contributions via an indirect femtosecond
time measurement encoded in the kinetic energy release (KER)
spectra for the D+ + D+ final state. Whereas the direct channel
ends at a high energy on the 1/R repulsive Coulomb potential
curve (where R is the internuclear separation), namely at the
equilibrium distance of the neutral molecule, leading to large
KERs, sequential ionization, especially from the 1sσg ground
state, might result in quite low fragment energies depending
on the internuclear separation (i.e., on the time when the
second photon was absorbed). Comparing the experimental
results to calculations on different levels of approximation
we can extract the relative contribution of direct and SI
channels, respectively. We further explore the R-dependent
ionization probability of D2 + in higher vibrational states,
study anisotropies in the fragment-ion angular distributions
©2010 The American Physical Society
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Rev. A 81, 051402(R) (2010)
343
RAPID COMMUNICATIONS
PHYSICAL REVIEW A 81, 051402(R) (2010)
Investigating two-photon double ionization of D2 by XUV-pump–XUV-probe experiments
Y. H. Jiang,1 A. Rudenko,2 J. F. Pérez-Torres,3 O. Herrwerth,4 L. Foucar,2 M. Kurka,1 K. U. Kühnel,1 M. Toppin,1 E. Plésiat,3
F. Morales,3 F. Martı́n,3 M. Lezius,4 M. F. Kling,4 T. Jahnke,5 R. Dörner,5 J. L. Sanz-Vicario,6 J. van Tilborg,7 A. Belkacem,7
M. Schulz,8 K. Ueda,9 T. J. M. Zouros,10 S. Düsterer,11 R. Treusch,11 C. D. Schröter,1 R. Moshammer,1 and J. Ullrich1,2
1
Max-Planck-Institut für Kernphysik, D-69117 Heidelberg, Germany
Max-Planck Advanced Study Group at CFEL, D-22607 Hamburg, Germany
3
Departamento de Quı́mica C-9, Universidad Autónoma de Madrid, 28049 Madrid, Spain
4
Max-Planck-Institut für Quantenoptik, D-85748 Garching, Germany
5
Institut für Kernphysik, Universität Frankfurt, D-60486 Frankfurt, Germany
6
Instituto de Fı́sica, Universidad de Antioquia, Medellı́n, Colombia
7
Lawrence Berkeley National Laboratory, Berkeley, California 94720, USA
8
Missouri University of Science & Technology Rolla, Missouri 65409, USA
9
Institute of Multidisciplinary Research for Advanced Materials, Tohoku University, 980–8577 Sendai, Japan
10
Department of Physics, University of Crete, Post Office Box 2208, 71003 Heraklion, Crete, Greece
11
DESY, D-22607 Hamburg, Germany
(Received 25 February 2010; published 24 May 2010)
2
We used a split-mirror setup attached to a reaction microscope at the free-electron laser in Hamburg (FLASH)
to perform an XUV-pump–XUV-probe experiment by tracing the ultrafast nuclear wave-packet motion in the
D2 + (1sσg ) with <10 fs time resolution. Comparison with time-dependent calculations shows excellent agreement
with the measured vibrational period of 22 ± 4 fs in D2 + , points to the importance of accurately knowing the
internuclear distance-dependent ionization probability, and paves the way to control sequential and nonsequential
two-photon double-ionization contributions.
DOI: 10.1103/PhysRevA.81.051402
PACS number(s): 33.80.Eh
Remarkable developments in femtosecond laser technology
have significantly advanced our understanding of ultrafast
processes in physics and chemistry [1]. Among the recent,
most fascinating achievements are few-cycle laser pulses
in the near infrared (IR), now implemented in pump-probe
schemes imaging the subfemtosecond nuclear wave-packet
(WP) motions in molecules [2,3]. In addition, the feasibility
of localizing electrons in simple molecular reactions has been
demonstrated by controlling the carrier envelope phase of such
fields [4]. Other advances include the generation of attosecond
[extreme ultraviolet (XUV)] pulses that deliver unique insight
in nuclear and electronic dynamics in molecules [5–7] in
IR-XUV (or vice versa) pump-probe experiments.
In this Rapid Communication we report the realization of
a femtosecond XUV-pump–XUV-probe scheme that opens
a new chapter in ultrafast science by exploiting the huge
flux of about 1012 photons/pulse of the free-electron laser at
Hamburg (FLASH). We trace in real time the femtosecond
nuclear WP dynamics in a prototype system, the 1sσg ground
state of D2 + , populated with about 95% by absorption of one
photon (38 eV) from the pump pulse as depicted in Fig. 1(a).
The dynamics is captured by the time-delayed XUV probe
pulse which “sequentially” ionizes D2 + and results in its
Coulomb explosion with the kinetic energy release (KER)
of the fragments, both measured in the reaction microscope
(REMI), and is proportional to the inverse of the internuclear
distance (R) at the instant of the second ionization.
The D2 molecule has been chosen because of its prototype
character for exploring the interplay between electronic and
nuclear motion in two-photon double ionization (TPDI) and
the availability of sophisticated calculations. TPDI of H2 /D2
has sparked considerable interest in theory just recently [8,9]
and pioneering experiments using single XUV pulses have
1050-2947/2010/81(5)/051402(4)
051402-1
been reported [10,11]. In Ref. [11] at 38-eV photon energy,
we demonstrated in a combined experimental and theoretical
investigation that “sequential” (involving real intermediate
states) and “direct” (via virtual levels) TPDI pathways can
be separated by measuring the KER of the D+ + D+ fragments. By making use of the inherent nuclear motion of
the vibrationally excited molecular ion, initiated by the first
ionization step, contributions at lower KERs were related
to sequential double ionization since the WP has moved to
larger distances by the time the second photon is absorbed. A
quantitative determination of the time interval between both
photoabsorption events, however, was not possible.
Here, together with model calculations, we trace the WP
motion and, thus, the absorption of the second photon in TPDI
in real time. By choosing specific time delays we are able to
select instances in time for the second ionization step where
the D2 + nuclear WP is either close to the outer or the inner
classical turning point in the bound 1sσg potential curve. This
way we extract information about the R dependence of the
ionization probability and, in comparison with theory, it points
to future ways to extract absolute direct TPDI cross sections.
Our experimental setup at FLASH [12] is comprised of
a REMI [13] equipped with an on-axis backreflection splitmirror setup for focusing and pulse-pair creation. In contrast to
already existing pulse-splitting schemes based on broad-band
grazing incidence mirrors [14,15], our setup consists of a
spherical multilayer mirror (1-in. Mo/Si mirror, 50-cm focal
length, <10-µm focus diameter) that is cut into two identical
“half-mirrors” (so-called “half-moon” geometry). The mirror
has a reflectivity of 40%, sharply peaked around 38 eV so that
higher-order harmonic radiation from the FEL is efficiently
suppressed. While one half-mirror is mounted at a fixed
position, the other one is movable along the FEL beam axis by
©2010 The American Physical Society
Max-Planck-Institut für Quantenoptik · Progress Report 2009/2010
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