Download 7 NCCR MUST

7 NCCR MUST
Friday, 22.06.2012, HPH G 2
Time
ID
MUST I
Chair: Lukas Gallmann, ETH Zürich
11:00
701
Probing electronic valence shell dynamics in molecules
Hans Jakob Wörner, Department Chemie, ETH Zürich
The structure and most properties of molecules are determined by their valence electrons. Understanding the correlated motion of these electrons and their dynamical rearrangements requires experimental methods with temporal resolution in the range of femtoseconds to attoseconds. We will
report on our recent progress in using high-harmonic generation (HHG) to characterize the electronic
structure and dynamics of molecules. Using recently developed techniques of impulsive orientation,
we demonstrate the observation of both even and odd high harmonics, characteristic of molecules
without inversion symmetry, and rationalize the results by comparison with ab initio electronic structure calculations. We report on the development of an impulsive Raman scattering scheme for the
preparation of electronic coherence in a molecule and its detection using HHG. Finally, we will show
how HHG can be used to probe electronic dynamics during photochemical reactions.
11:30
702
Electron ionization times measured with the attoclock
Robert Boge 1, Claudio Cirelli 1, Adrian N. Pfeiffer 1, Mathias Smolarski 1, Reinhard Dörner 2,
Ursula Keller 1
1
Department Physik, ETH Zürich, Wolfgang-Pauli-Str. 16, 8093 Zürich
2
Institut für Kernphysik, Johann Wolfgang Goethe Universität, Max-von-Laue-Str. 1,
DE-60438 Frankfurt am Main
With the attoclock technique we have direct access to electron ionization timing in helium and argon
exposed to a strong laser field in the near infrared. So far we could put an upper limit to the tunneling
delay time of a few tens of attoseconds - much shorter than the predicted Keldysh tunneling time.
More recently we also resolved the time delays of the two electrons in strong-field double ionization
of Argon. In the attoclock, the magnitude of the electron momenta follows the envelope of the laser
pulse and gives a coarse timing for the electron releases (like an hour hand of a clock), while the fine
timing (the minute hand) is provided by the emission angle of the electrons. The results show that the
time difference between the two ionization events is significantly smaller than predicted by a semiclassical simulation where the two electrons are treated independently.
11:45
703
Attosecond Time-Gated Absorption and Emission
Jens Herrmann 1, Mazyar Sabbar 1, Reto Locher 1, Matthias Weger 1, P. Rivière 2, U. Saalmann 3,
J.-M. Rost 3, Lukas Gallmann 1, Ursula Keller 1
1
Departement Physik, ETH Zürich, Wolfgang-Pauli-Strasse 16, 8093 Zürich
2
Departamento de Química, Universidad Autónoma de Madrid, ES-28049 Madrid
3
Max-Planck Institute for the Physics of Complex Systems, DE-01187 Dresden
The transmission of electro-magnetic radiation through an off-resonant optical medium is one of the
most common processes in nature and is not expected to hold any surprises. We show that the passage of a short and off-resonant electro-magnetic radiation pulse intrinsically involves transient absorption and emission on the time-scale of the carrier wave oscillations. For the usual time-integrated
observation, no net effect survives. We show, however, that an additional control field modulating the
energy of a nearby resonance can time-gate this process and result in macroscopic net emission and
absorption. We experimentally demonstrate the effect with an attosecond extreme ultraviolet (XUV)
pulse passing off-resonantly through helium gas in the presence of an infrared control field. The delay
between the XUV and infrared pulses controls the occurrence of either absorption or emission. The
strength of maximum emission is of the same order as the maximum induced absorption and thus
hints at a new mechanism for achieving optical gain in the XUV spectral region.
12:00
Postersession (continued), Lunchbuffet
Public Tutorial see p. 21
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Time
ID
MUST II
Chair: Thomas Feurer, Uni Bern
13:30
711
Optimal Dynamic Discrimination of Free Amino Acids and Small Peptides
Jean-Pierre Wolf 1, Luigi Bonacina 1, Svetlana Afonina 1, Ariana Rondi 1, Christoph Hauri 2,
Alexandre Trisorio 2
1
GAP, University Geneva, 22 ch de Pinchat, 1211 Genève, 2 Paul Scherrer Institut, 5232 Villigen PSI
Coherent manipulation of molecular wavepackets in biomolecules might contribute to the quest towards label-free cellular imaging and protein identification. We report the use of optimally tailored
UV laser pulses in pump-probe depletion experiments that selectively enhance or decrease fluorescence between two aromatic amino acids: tryptophan (Trp) and tyrosine (Tyr). Selective fluorescence
modulation is achieved with a contrast of > 35%. The mechanism invoked is the transient modulation of the S1-Sn transition by the optimally shaped pulse, as compared to the reference pulse. The
discrimination capability was extended to di-peptides, i.e. alanine-, glycine-, and leucine-tryptophan.
For several molecule pairs (trp vs dipeptides, gly-trp vs ala-trp) the discrimination capability of the
approach is the order of 20%, which is remarkably high considering the modest bandwidth available
at 266 nm (3 nm FWHM). This restricted parameter space does not allow, in opposite, to discriminate
other dipeptides like ala-trp and leu-trp.
13:45
712
Dynamic probe concept for studying aggregation of organic dye molecules at
liquid/liquid interfaces by femtosecond second harmonic generation technique
Marina Fedoseeva, Eric Vauthey
Physical Chemistry Department, University of Geneva, 30, Quai Ernest Ansermet, 1211 Geneva
Aggregation of organic dye molecules at liquid/liquid interfaces was studied by time-resolved and
stationary second harmonic generation (SHG), whose interfacial selectivity makes it a powerful tool
for getting direct access to the ultrafast dynamics and chemical composition at interfaces. The use
of the SHG technique was combined with the so-called dynamic probe concept where the dynamic
probe is a molecule, whose photophysics depends on the environmental properties such as local
friction, salt concentration, hydrogen bonding, etc. In the present study such dynamic probes were
organic dye molecules, which were adsorbed at the interface creating a certain charge distribution.
Addition of salts or surfactants strongly affected this charge distribution through the electrostatic
interactions, which in some cases resulted in stronger dye adsorption and further formation of dye
aggregates. This was followed by a dramatic change of the excited-state dynamics of a dye molecule
and shifts in its interfacial spectra.
14:00
713
Breaking Down the Problem to Understand the Photophysics
of Conjugated Polymers
Natalie Banerji 1, Mariateresa Scarongella 1, Jonathan Yuen 2, Andrey Laktionov 3, Mario Leclerc 4,
Fred Wudl 2, Ursula Röthlisberger 3, Jacques-Edouard Moser 1
1
EPFL SB ISIC GR-MO, Station 6, 1015 Lausanne
2
University of California, Center for Polymers and Organic Solids, 931065090 Santa Barbara, USA
3
EPFL SB ISIC LCBC, 1015 Lausanne
4
Université Laval, Department of Chemistry, CA-Quebec City, G1K 7P4
Semiconducting polymers have evolved as an extremely important class of materials for organic
electronics. Their properties are largely determined by the extended electron delocalization in the
conjugated systems, but the influence of the individual building blocks making up the polymer chain
should not be underestimated. Polymers with electron-donating and electron-withdrawing units in
their structure (push-pull effect) are of particular interest for applications. For example PCDTBT, which
yields high power conversion efficiency in organic solar cells, consists of alternating carbazole donor
groups and dithienyl-benzothiadiazole (BT) acceptor groups. To understand the complex photophysics of this material, we have broken it down and studied the intrinsic properties of the separate donor,
the acceptor and the donor-acceptor dimer using (time-resolved) spectroscopy and DFT calculations.
[1] A particular emphasis of our investigation was the charge transfer character and electron delocalization in the excited state. Another related acceptor group, dithienyl-benzobisthiadiazole (BBT)
is found in polymers with ambipolar charge transport in field effect transistors. [2] By combining
spectroscopy and electron spin resonance measurements, we have identified the singlet open-shell
character in the biradicaloid ground state of isolated BBT as possible origin of the ambipolarity.
[1] N. Banerji et al., submitted for publication.
[2] J. D. Yuen et al., Adv. Mater. (2011), 23, 3780.
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14:15
714
Investigation of low frequency vibrations using dispersed femtosecond – DFWM
Gregor Knopp 1, Peter Radi 1, Yuzhu Liu 1, Luca Castiglioni 2, Thomas Gerber 1
1
Paul Scherrer Institut, Molecular Dynamics, 5232 Villigen PSI
2
Surface physics, University Zürich, Winterthurerstr. 190, 8057 Zürich
Low frequency modes, such as bending or torsion motions of large molecules appear in the Terahertz
frequency regime with energies less than 300 cm-1. Unfortunately, due to usually weak dipole and Raman transition moments, these motions are difficult to address experimentally. The high peak-power
output of fs-lasers, facilitates the excitation of weak Raman coherences and in nonlinear spectroscopy, the amplitude of a signal can be effectively enhanced by incrementing the field strength of the
interacting laser pulses. Fs-degenerated four wave mixing (fs-DFWM) allows the excitation of Raman
coherences within the bandwidth of the applied laser pulses. Aside from this, intense laser pulses
can cause auxiliary signal contributions due to fifth order scattering and ionization processes. Such
interfering terms modify the experimental results and signals, detected in the time-frequency domain,
contain supplementary information. Dispersed fs-DFWM is used to investigate the dynamics of the
bending and torsion motions in the electronic ground state of ethylbenzene.
14:30
715
Multidimensional IR spectroscopy of water
Peter Hamm, Uni Zürich
We will present multidimensional IR spectroscopy of both the OH stretch vibration of water (2D IR and
3D IR spectroscopy), as well as of its inter-molecular degrees of freedom by 2D-RamanTHz. Thee IR
spectrum of water, that spans the frequency range from 50 cm-1 to 3000 cm-1, reports on the constant
forming, breaking and rearranging of the hydrogen bond network of water, which is responsible for
its peculiar properties. In contrast to just IR absorption spectroscopy (i.e.; 1D spectroscopy), multidimensional spectroscopies may elucidate correlations in these processes. The experimental work is
supplemented by molecular dynamics simulations.
15:00
716
Measuring nonadiabaticity of molecular quantum dynamics with quantum fidelity
and with its efficient semiclassical approximation
Tomáš Zimmerman, Jiri Vanicek
Laboratory of theoretical physical chemistry, EPFL SB SCGC-GE, 1015 Lausanne
We propose to measure nonadiabaticity of molecular quantum dynamics rigorously with the quantum
fidelity between the Born-Oppenheimer and fully nonadiabatic dynamics. It is shown that this measure of nonadiabaticity applies in situations where other criteria, such as the energy gap criterion or the
extent of population transfer, fail. We further propose to estimate this quantum fidelity efficiently with
a generalization of the dephasing representation to multiple surfaces. Two variants of the multiplesurface dephasing representation (MSDR) are introduced, in which the nuclei are propagated either
with the fewest-switches surface hopping or with the locally mean field dynamics (LMFD). The LMFD
can be interpreted as the Ehrenfest dynamics of an ensemble of nuclear trajectories, and has been
used previously in the nonadiabatic semiclassical initial value representation. In addition to propagating an ensemble of classical trajectories, the MSDR requires evaluating nonadiabatic couplings
and solving the Schrödinger (or more generally, the quantum Liouville-von Neumann) equation for a
single discrete degree of freedom. The MSDR can be also used in the diabatic basis to measure the
importance of the diabatic couplings. The method is tested on three model problems introduced by
Tully and on a two-surface model of dissociation of NaI.
15:15
717
Perturbative Treatment of the Up-Conversion Detection
of Pulse-shaped Entangled Photons and Applications
Christof Bernhard, Bänz Bessire, Thomas Feurer, Andre Stefanov
Institute for Applied Physics, University of Bern, Sidlerstr. 5, 3012 Bern
We introduce and describe sum frequency generation of broadband time-energy entangled photons
in a non-linear crystal as an ultrafast coincidence detection method. Further, the optical transfer function of an arbitrary optical setup has been implemented in the formalism. Based on this, we show
that such entangled photon pairs can be shaped very similar to bradband ultrashort light pulses.
First, we use the optical transfer function of a two path interferometer implemented on a spatial light
modulator (SLM) to investigate bi-photon interference by simultaneously shaping the amplitude and
phase of the two-photon wave function. We show that the interference pattern is best described by a
correct quantum mechanical description of the up-conversion process. Due to the phase sensitivity
of the coincidence signal we then reconstruct the phase of the bi-photon state using an optimization
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algorithm. Second, we propose and demonstrate the ability to encode qudits in the energy spectrum
of entangled photon. Entangling higher dimensional bipartit systems (qudits) has been shown to
give more insight on the nature of entanglement compared to the simplest entanglement system
composed of two two-level systems (two qubits). Experimentally, higher dimensional entanglement
in photonic systems has been demonstrated for various degrees of freedom of light. Entangled qudits
can be implemented in the timing of entangled photons, in the transverse momentum, or in orbital
angular momentum modes. We show experimental violation of Bell inequalities for qubits and qutrits
and study the violation of Bell inequalities as a function of the entanglement.
15:30
Coffee Break
MUST III
Chair: Jürg Osterwalder, Uni Zürich
16:00
721
High-harmonic generation from oriented OCS molecules
Peter Kraus, Stefan Vlajkovic, Alisa Rupenyan, Hans Jakob Wörner
LPC, ETH Zürich, Wolfgang-Pauli-Strasse 10, 8093 Zürich
We report even-order high-harmonic generation (HHG) from oriented OCS molecules. Experimentally
we use the output of a Ti:Sa regeneratively amplified laser system. For orienting molecules one part
of the output is frequency doubled and the group delay between fundamental and second harmonic
is adjusted with a calcite plate. The other part is used as probe for HHG. Oriented molecular samples
have the unusual property of generating even harmonics as a consequence of the anisotropy of the
generation medium, which we can observe in our measurements. Our results show good agreement
with quantum scattering calculations. Furthermore, coherent control of the even harmonic emission
was achieved by tilting the calcite plate thus adjusting the phase delay of fundamental and second
harmonic. Performing measurements in the molecular frame is a requirement for further studies aiming at probing chemical reactions and imaging of molecular charge migration on the attosecond time
scale in polar molecules.
16:15
722
A double-sided time-resolved VMI setup with high temporal resolution
Yuzhu Liu, Gregor Knopp, Thomas Gerber, PSI, Molecular Dynamics, 5232 Villigen PSI
We assemble a double-sided time-resolved VMI setup, simultaneously collecting energy and momentum information about electrons and coincident ions. We intend to preserve the longitudinal focus
condition for TOF mass resolution on our setup while maintaining the lateral focus for optimal VMI
energy resolution simultaneously, which can be achieved by the implementation of additional, especially optimized, electrical lenses. By simulations (Simion 8.1), we can demonstrate that it is possible
to achieve velocity-resolving capabilities of <1% (full range regime) in conjunction with a high temporal resolution for TOF mass spectrum. Compared with a conventional VMI setup, the mass resolving
power of the new design exceeds the conventional one by almost ten times.
16:30
723
Femtosecond dynamics of atomic structure in solids
Steve L. Johnson, Ultrafast Dynamics Group, Institute of Quantum Electronics, ETH Zürich
Recent developments in generating ultrashort pulsed x-ray sources have enabled a new class of
experimental investigations on the dynamics of atomic-scale structure in solid state materials. The
short wavelength of x-rays combined with short pulse durations allow for diffraction experiments that
directly probe changes inatomic scale structure. For lattice motion, the fundamental time scale of
interest for dynamics is the period of lattice vibrations, the fastest of which are tens of femtoseconds.
For the structure of electronic states (charge, orbitals) and magnetic moments the time scales of
excitation can be even faster, extending into the attosecond regime. The ability to resolve dynamics
on time scales approaching these values allows for both a better understanding of coupling among
excitations in the time domain as well as new possibilities for the fast manipulation of structure using
strongly non-equilibrium pathways. This has potentially important implications in schemes for fast
data processing and storage. In this talk I will give a brief overview of how time resolved x-ray techniques can be used to understand the physics and the limiting time scales of long-range structural
changes in materials, using specific examples of structural dynamics in charge density wave systems
to spin order dynamics in antiferromagnetic CuO. These examples show both the versatility and the
specificity of short-pulse x-ray based methods in understanding the non-equilibrium dynamics of
strongly excited materials.
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Time
ID
Chair: Paul Beaud, PSI Villigen
17:00
724
Femtosecond Transient Diffuse Reflectance for Dye-Sensitized Solar Cells
Elham Ghadiri, Jacques-E. Moser
Institute of chemical sciences & engineering, EPFL SB ISIC GR-MO Station 6, 1015 Lausanne
Diffuse reflectance spectroscopy is a technique that permits optical spectroscopic measurements of
highly scattering samples. We have applied femtosecond diffuse reflectance spectroscopy technique
to study the electron injection process on a dye sensitized solar cell photoanode. The charge carrier
dynamics of TiO2 films made of different size of nanoparticles covered with one monolayer of dye
are investigated. In this configuration, a first short laser pulse (600 nm, dye ground state absorption)
excites the sample, while a second time-delayed and spatially coincident short laser pulse (850 nm,
dye oxidized state absorption) strikes the sample, diffuse scattered light of probe beam is then collected. The transient absorption change can be measured and corrected by determining the absolute
amount of diffuse reflected light with and without pump beam. The results show that the photo- electrons back reaction rate with oxidized dye is increased in case of large particles of 400 nm respect
to 20 nm particles.
17:15
725
p-Conjugated Donor-Acceptor Systems as Metal-Free Sensitizers
for Dye-Sensitized Solar Cell Applications
1
Mateusz Wielopolski 1, Jacques-E. Moser 1, Shaik M. Zakeeruddin 2, Michael Grätzel 2
Institute of Chemical Sciences & Engineering, EPFL, SB ISIC GR-MO, Station 6, 1015 Lausanne
2
Laboratory of Photonics and Interfaces, EPFL, SB ISIC LPI, Station 6, 1015 Lausanne
Metal-free organic sensitizers for dye-sensitized solar cells exhibit lower photoconversion efficiencies (< 10 %) than corresponding ruthenium complexes (11.7 %) but they can be synthesized rather
inexpensively and their optical properties can be tuned by molecular design. Among these, structures
implementing p-conjugated donor-acceptor (D-p-A) architectures exhibit growing interest due to the
tunability of their electronic properties by their conjugation length. Ultrafast techniques such as femtosecond and nanosecond transient absorption spectroscopy allow studying the dependence of the
charge carrier dynamics in various organic dye/TiO2 systems on parameters such as donor-acceptor
distance, p-conjugation length and coupling to TiO2 by different anchoring groups. We have shown
that a sensitive interplay between the dyes’ building blocks governs the charge injection/recombination and dye regeneration rate constants and they do not necessarily follow an exponential distance
dependence as given by kCT = k0x exp(-brDA) and often stated for dyes of D-p-A architecture.
17:30
726
Probing interfacial electron transfer dynamics in the attosecond time domain
Luca Castiglioni 1, Michael Greif 1, Dominik Leuenberger 1, Matthias Hengsberger 1,
Jürg Osterwalder 1, Reto Locher 2, Lukas Gallmann 2, Ursula Keller 2
1
Physik-Institut, Universität Zürich, Winterthurerstrasse 190, 8057 Zürich
2
ETH Zürich, Wolfgang-Pauli-Strasse 16, 8093 Zürich
Electron dynamics at surfaces and interfaces are of fundamental importance in chemical reactions,
heterogeneous catalysis and photovoltaic devices. Noble metals coated with diamondoid SAMs exhibit a nearly monochromatic photoelectron spectrum, which is attributed to ultrafast relaxation of
hot electrons in the metal and efficient interfacial electron transfer with subsequent emission from
the molecule LUMO.
We present both simulations and first experimental results of an attosecond streaking experiment to
directly probe these electron dynamics in the time domain. Using an s-polarized streaking field leads
to time-dependent transversal acceleration of photo-emitted electrons. The change in emission angle
as a consequence of the imparted momentum can be measured in an angle-resolved photoelectron
spectrometer. Preliminary experiments show that above-threshold photoemission (ATP) can be significantly reduced using s-polarization. The simulations suggest that the time-resolution compares
well to that of previous attosecond streaking experiments and that we can resolve temporal features
on the order of 100 as up to 10 fs.
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17:45
727
Atomic motion of a coherent phonon observed
in a charge and orbitally ordered manganite
Andrin Caviezel 1, Paul Beaud 1, Simon Mariager 1, Gerhard Ingold 1, Urs Staub 2,
Marios Garganourakis 2, Shih-Wen Huang 2, Chris Milne 3, Sang-Wook Cheong 4, Steven Johnson 5
1
FEMTO Gruppe - 2 RESOXS Gruppe, Paul Scherrer Institut, 5232 Villigen PSI
3
Physik Departement, EPF Lausanne, 5232 Villigen PSI
4
Dep. of Physics & Astronomy, Rutgers Univ., 136 Frelinghuysen Road, Piscataway 08854 NJ, USA
5
Physik Departement, ETH Zürich, Wolfgang-Pauli-Strasse 16, 8093 Zürich
La0.25Pr0.375Ca0.375MnO3 belongs to the three dimensional transition metal oxides of the form R1-xAxMnO3
and exhibits charge and orbital order (CO/OO) with a doubled unit cell and a structural superlattice
below TCO  220 K. A low frequency coherent Ag phonon serves as signature for this CO/OO state.
This phonon initially assigned to the rotation of the Mn-O6 octahedra [1] was later attributed to the
motion of the R/A cations because of its independency on oxygen isotope substitution [2]. However,
our latest time resolved x-ray diffraction measurements on several superlattice peaks with different
sensitivity to the atomic motion of the unit cell constituents favour the intial picture.
[1] M. N. Iliev et al., PRB 57, 2872 (1998)
[2] V. A. Amelitchev et al., PRB 63, 104430 (2001)
18:00
728
Electron dynamics in a quasi-1-dimensional topological metal: Bi(114)
Matthias Hengsberger, Dominik Leuenberger, Michael Greif, Luca Castiglioni, Jürg Osterwalder,
Physics Institute, University of Zürich, Winterthurerstrasse 190, 8057 Zürich
In heavy elements, spin splittings of surface electronic states are large due to strong spin-orbit interaction [1]. Recently, a topological metal was discovered on a Bi surface, Bi(114), which exhibits
1-dimensional atomic rows [2]. The scattering rates are low due to the low density of states at the
Fermi level and the spin splitting of the surface states, resulting in long lifetimes. Using time-resolved
photoemission data, we demonstrate that the scattering rates of unoccupied bulk states depend on
the pump fluence due to the occupation of available scattering final states by hot electrons. Spectra
of the long-lived hot electron gas exhibit energy and intensity oscillations due to the excitation of
coherent phonon modes, amongst which one is directly related to this special 1-dimensional surface.
[1] J. H. Dil, J. Phys.: Condens. Matter 21, 403001 (2009).
[2] J. Wells et al., Phys. Rev. Lett. 102, 096802 (2009).
18:15
729
Laser induced coherent structural dynamics of the Heusler alloy Ni2MnGa
Simon O. Mariager, Andrin Caviezel, Paul Beaud, Christoph Quitmann, Gerhard Ingold
Swiss Light Source, PSI, 5232 Villigen PSI
Heusler alloys exhibit interesting electronic and multiferroic phenomena. Members of the half Heusler
family are predicted to be topological insulators and half-metal full Heusler alloys exhibit high spin polarization suitable for spintronic applications or magnetic shape memory (MSM) effects for magnetomechanical and magneto-caloric applications. Ni2MnGa is the prototype MSM alloy where the combination of ferromagnetism and a martensitic phase transition allows field induced strains of 10%.
While technical applications exist, the microscopic origin of the phase transition is not understood.
Using optical pump-probe experiments we have identified two optical phonon modes in the 1 THz
range for the first time. These phonons are clearly related to the phase diagram and the martensite
and pre-martensite phases of Ni2MnGa. Static x-ray diffraction studies of the structure hints to the
origin of these phonons, but only future time-resolved x-ray experiments can reveal their true nature
and their relation to the magneto-structural phase transition.
119
18:30
730
Non-retarded pairing interaction in a high-Tc cuprate
from coherent charge fluctuation spectroscopy
Fabrizio Carbone, EPFL
Dynamical information of spin degrees of freedom of proteins or solids can be obtained by nuclear
magnetic resonance (NMR). A technique with similar versatility for charge degrees of freedom and
their ultrafast correlations could move forward the understanding of systems like high-Tc cuprates.
By perturbing the superconducting state in a cuprate using a femtosecond laser pulse we generate
coherent oscillations of the Cooper pairs which can be described by NMR-like equations. The oscillations are detected by transient broad band reflectivity and found to resonate at the typical scale of
Mott physics (2.6 eV) implying a substantial contribution of non-retarded interactions to the pairing,
as in unconventional (non Migdal-Eliashberg) theories.
19:00
END
ID
MUST Poster
741
Direct High Harmonics Pulse Shaping in the XUV
Jean-Pierre Wolf , Denis Kiselev 1, Luigi Bonacina 1, Stefan Vlajkovic 2, Peter Kraus 2, Hans-Jakob Wörner 2
1
GAP, University Geneva, 22ch de Pinchat, 1211 Geneva
2
Laboratorium für Physikalische Chemie, ETH Zürich, 8093 Zürich
1
Direct amplitude and phase control of high harmonics (HH) and attosecond pulses would open new perspectives in the coherent manipulation of atomic and molecular electrons at ultimate timescales. In this contribution,
we present the first experiment demonstrating direct pulse shaping in the XUV (14-36 eV) of individual HH,
independently. The experiment is based on a MEMS-based reflective device that we recently developed with
the IMT (Neuchatel), providing 100 micromirrors. The HH produced in a gas jet are dispersed and focused by a
toroidal XUV grating onto a microchannel-plate (MCP) detector after reflection on the MEAs each harmonic is
spatially spread on several pixels, amplitude modulation is achieved gradually by tilting an increasing number
of micromirrors. Note that instead of amplitude modulation, the tilt motion can also be used without mask for
achieving k vector optimization in multi-XUV wave mixing experiments.
742
High-Power Mid-infrared Femtosecond Laser Source Based on Parametric Transfer
C. Heese 1, B. W. Mayer 1, C. R. Phillips 2, L. Gallmann 1, M. M. Fejer 2, U. Keller 1
1
ETH Zürich, Wolfgang-Pauli-Strasse 16, 8093 Zürich
2
Standford University, Edward L. Ginzton Laboratory, 94305 Stanford, USA
We present a compact, ultra-broadband laser source producing femtosecond pulses in the mid-infrared (MIR).
Our approach is to parametrically generate the MIR radiation by optical parametric amplification (OPA). For this
we use a commercial femtosecond seed laser operating at 1.56 µm wavelength and an industry-grade picosecond pump laser centered at 1 µm. The spectral phase of the MIR pulses can be manipulated with a pulse shaper
through parametric transfer from the near-infrared (NIR) seed. With this system we achieve clean 75-fs pulses
(~7 optical cycles) centered at 3.4 µm wavelength with a pulse energy of 7 µJ and a repetition rate of 100 kHz.
These parameters result in 93 MW peak power. Such a long-wavelength few-cycle laser source is particularly
interesting for the investigation of strong-field laser-matter interaction, which strongly scales with wavelength.
743
Stereochemistry of C4 dicarboxylic acids on Cu(110)
Chrysanthi Karageorgaki, Karl-Heinz Ernst, EMPA, Überlandstrasse 129, 8600 Dübendorf
In order to better understand self assembly on the molecular level, we are currently studying different chiral butanedioic acids, like tartaric acid (TA), malic acid (MA) and 2,3-dimethyl succinic acid (DMSU), as well as achiral
analogues like succinic acid (SU), maleic acid (MEA), meso-DMSU and trans 1,2-cyclohexane dicarboxylic acid
(CHDCA) on a Cu(110) surface. Here we present LEED, XPS and TPD results of the aforementioned compounds.
Apart from coverage – dependent "surface explosion" decomposition, all achiral compounds undergo symmetry breaking, observed as superposition of mirror domains in LEED. Interestingly enough, the obtained LEED
structures coincide for the various compounds, suggesting that the chiral recognition at surfaces derives from
the substrate, when the respective molecule doesn’t have a functional group able to form strong intermolecular
bonds.
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744
Beating the efficiency of both quantum and classical simulations with semiclassics
Cesare Mollica, Jirí Vanícek, EPFL, 1015 Lausanne
While rigorous quantum dynamical simulations of many-body systems are extremely difficult (or impossible) due
to the exponential scaling with dimensionality, corresponding classical simulations completely ignore quantum
effects. Semiclassical methods are generally more efficient but less accurate than quantum methods, and more
accurate but less efficient than classical methods. We find a remarkable exception to this rule by showing that
a semiclassical method can be both more accurate and faster than a classical simulation. Specifically, we prove
that for the semiclassical dephasing representation the number of trajectories needed to simulate quantum fidelity is independent of dimensionality and also that this semiclassical method is even faster than the most efficient
corresponding classical algorithm. Analytical results are confirmed with simulations of quantum fidelity in up to
100 dimensions with 21700-dimensional Hilbert space.
745
Confocal fs-CARS measurement of nano-particles in epi-direction
Gregor Knopp 1, Yaroslav Sych 1, Yuzhu Liu 1, Peter Radi 1, Anastasija Ichsanow 2, Thomas Gerber 1
1
Paul Scherrer Institut, Molecular Dynamics, 5232 Villigen PSI
2
Paul Scherrer Institut, X-FEL, 5232 Villigen PSI
The structure of model catalyst surfaces deviates from that of technical catalysts, especially when the particles
have a nanometer size. The ability of nanostructures to enhance catalytic efficiency compared eith the bulk
material leads to an increasing interest in these systeConfocal anti-Stokes Raman (CARS) microscopy, in which
the interacting fields are spatially confined to a tight (µm) focal spot, has a great potential for investigations of
time-resolved dynamics of electronic and vibrational excitations of nanostructures. CARS generation, however,
originating from regions near the interface can also contribute to the signal. By resonant enhancement we intend
to yield signals that arise from CO adsorbed on the Pt nano-structures. Due to localized surface plasmons an
additional enhancement of the nonlinear response from Pt nano-structures is expected. We present fs-CARS
measurements in epi-direction derived from a slurry of alumina and pt nanoparticles, deposited on a silica support.
746
Probing the longitudinal momentum spread of the electron wave packet at the exit point
Alexandra Landsman 1, Adrian Pfeiffer 1, Claudio Cirelli 1, Matthias Smolarski 1, Darko Dimitrovski 2,
Lars Madsen 2, Ursula Keller 1
1
Institute of Quantum Electronics, ETHZ, Wolfgang-Pauli-Str. 16, 8093 Zürich
2
Aarhus University, Department of Physics, Aarhus C, DK-8000 Aarhus
We present an ellipticity resolved study of momentum distributions arising from strong-field ionization of Helium
at constant intensity. The influence of the ion potential on the departing electron is considered within a semiclassical model consisting of an initial tunneling step and subsequent classical propagation. We find that the
momentum distribution can be explained by the presence of a longitudinal momentum spread of the electron
at the exit from the tunnel. Our combined experimental and theoretical study provides an estimate of this momentum spread.
747
Accelerating the calculation of time-resolved electronic spectra
with the cellular dephasing representation
Miroslav Šulc, Jiří Vaníček, Institut des sciences et ingénierie chimiques, EPFL, 1015 Lausanne
Dephasing representation of fidelity, also known as the classical phase averaging method, can be considered as
a special case of Miller's linearized semiclassical initial value representation and belongs among the most efficient approximate semiclassical approaches for the calculation of ultrafast time-resolved electronic spectra. Recently it has been shown that the number of trajectories required for convergence of this method is independent
of the system's dimensionality. Here we propose a further accelerated version of the dephasing representation in
the spirit of Heller's cellular dynamics. The basic idea of the "cellular dephasing representation" is to decompose
the Wigner transform of the initial state into a phase space Gaussian basis and then evaluate the contribution of
each Gaussian to the relevant correlation function approximately analytically, using numerically acquired information only along the trajectory of the Gaussian's center. The approximate nature of the DR classifies it among
semiclassical perturbation approximations proposed by Miller and Smith, and suggests its limited accuracy.
Yet, the proposed method turns out to be sufficiently accurate whenever the interaction with the environment
diminishes the importance of recurrences in the correlation functions of interest. Numerical tests on a collinear
NCO molecule indicate that even results based on a single classical trajectory are in a remarkable agreement
with the fully converged DR requiring approximately 104 trajectories.
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748
Towards femtosecond dynamics in multiferroics
Steven L. Johnson, Teresa Kubacka
Institute for Quantum Electronics ETHZ, Wolfgang-Pauli-Strasse 16, 8093 Zürich
Magnetoelectric coupling in induced multiferroics has drawn intense interest in recent years, and its microscopic
mechanisms are still a subject of active debate. Whereas near-equilibrium characteristics of these materials have
been thoroughly studied, there is much more to be learned by a systematic study of the dynamics of magnetoelectric coupling in a strongly non-equilibrium regime. The key element of novel ultrafast pump-probe experiments is the use of extremely intense sources of short-pulse electromagnetic radiation over a broad frequency
range, what allows for selective stimulation and observation of the structural dynamics. Such experiments offer
unique ways to gain important information about the energy-transfer processes in a particular system and thus
insight into the microscopic mechanisms governing its behavior. This poster investigates methods of studying
ultrafast phenomena in selected multiferroics. Additionally, possibilities of manipulation of the multiferroic phase
by resonant excitation of an electromagnon mode with coherent electromagnetic radiation are discussed.
749
Photon echo measurements
using a frequency doubled cavity dumped femtosecond oscillator
Vesna Markovic, Bernhard Lang, Eric Vauthey
Department of Physical Chemistry, University of Geneva, 30 Quai Ernest Ansermet, 1211 Geneva
The aim of the project is a detailed comparison of the photon echo technique and related population spectroscopy techniques based on transient absorption and time resolved fluorescence (TRF) detection for investigating
polar solvation dynamics as well as electron and energy transfer processes. The same laser system can be used
as a source for our photon echo set-up and an all-reflective up-conversion set-up that allows studying solvation
dynamics using the same sample under identical excitation conditions by two different experimental methods,
Three Pulse Photon Echo Peak Shift and TRF. Moreover, we are implementing the heterodyne detection scheme
which will allow two-dimensional electronic spectroscopy. Our experimental set-up and different population
spectroscopy techniques will be discussed.
750
A Combined NIR Transient-Absorption Optical Pump-THz Probe Spectroscopy Study on
Charge Carrier Generation Dynamics in Solid State Dye Sensitized Solar Cells
Jan Brauer, Arianna Marchioro, Jacques-E. Moser, ISIC, EPFL, Station 6, 1015 Lausanne
In this work we investigated the influence of infiltrating a dye-sensitized mesoporous TiO2 layer with the organic
hole-conducting material Spiro-OMeTAD on the kinetics of charge carrier photo-generation. The dynamics recorded by probing the appearance of free conduction band electrons by THz spectroscopy is quite different
from those obtained by monitoring the appearance of the oxidized states of the sensitizer by transient absorption spectroscopy. The effect of commonly used additives, such as tBP and Li+ salts, was also investigated. A
model is proposed, which involves an intermediate state due to electrostatic interaction between photo injected
electrons in the solid and dye-cations adsorbed at the surface.
751
Investigation of chemical surface treatment on the charge carrier dynamics in solid-state
Dye-Sensitized Solar Cells
1
Arianna Marchioro 1, Amalie Dualeh 2, Jacques-Edouard Moser 1
EPFL SP ISIC GR MO Station 6, 1015 Lausanne, 2 EPFL SP ISIC LPI Station 6, 1015 Lausanne
Solid-state Dye-Sensitized Solar Cells (ssDSCs) are a promising and challenging alternative to conventional
liquid DSCs. Up to now, the cell efficiency is increased by the addition of a specific fabrication step that modifies the metal-oxide semiconductor surface. However so far, its actual effect on the cell is still unclear. Based
on kinetic studies obtained by femtosecond and nanonsecond transient absorption spectroscopy, this surface
treatment was shown to affect all the dynamics of the carriers in the cell: electron injection, hole injection and
charge recombination. Herein, we propose a model describing the interaction at the interface between titanium
dioxide, dye and organic hole-conducting material Spiro-OMeTAD.
752
Photoinduced Processes of Small Molecule Organic Photovoltaics
1
Jelissa De Jonghe 1, Jacques-E. Moser 1, Frank Nüeusch 2, Gaetan Wicht 2, Roland Hany 2
Institute of Chemical Cciences and Engineering, EPFL SB ISIC GR-MO, Station 6, 1015 Lausanne
2
Funktionspolymere, EMPA, Überlandstr. 129, 8600 Dübendorf
The ultrafast charge separation processes occurring in bilayer small molecule organic photovoltaics are investigated via femtosecond transient absorption spectroscopy. The interface between donor and acceptor
122
molecules as well as their aggregation are of particular interest in order to determine the first steps of charge
separation. Molecules such as cyanine dyes are potential candidates as molecular donors due to their high
extinction coefficient, which enables the use of thin active layers and therefore enhances charge separation.
Those molecules exhibit charge separated states living up to the microsecond time scale, and the mechanism
of formation of those states is currently addressed.
753
Photoelectron Diffraction on SnPc/Ag(111)
Michael Greif, Luca Castiglioni, Jürg Osterwalder, Matthias Hengsberger
Physik-Institut, Universität Zürich, Winterthurerstrasse 190, 8057 Zürich
X-Ray- (XPD) and UV-photoelectron diffraction (UPD) are powerful methods to resolve structures of molecules
on surfaces. Here photoelectrons are emitted from the center tin atom of Tin-Phthalocyanine (SnPc) and are
coherently scattered by the neighboring atom. By measuring the photoelectron current in the half space above
sample the diffraction pattern can be recorded. SnPc as a non-planar molecule with two different conformations
serves as a tunable model system to test the sensitivity of UPD to larger molecules on surfaces. The comparison
with Single-Scattering-Calculations for SnPc/Ag(111) shows the flattening of the molecule and its azimuthal
orientation with respect to the crystal axes. UPD additionally yields information not only of the molecule but also
of the substrate due to strong backscattering. Exciting the molecules with pulsed laser light will induce dynamics
which can then be measured with pump and probe photoelecron spectroscopy.
754
Effects of the finite length of the pump laser pulse in nonadiabatic quantum dynamics
simulations of ultrafast time-resolved spectroscopy
Aurélien Patoz, Jiri Vanicek, Departement Chemistry, EPFL SB ISIC LCPT, 1015 Lausanne
We have implemented a general split-operator algorithm for exact nonadiabatic quantum dynamics of a molecule interacting with a time-dependent electromagnetic field. This algorithm enables evaluation of the accuracy
of the Franck-Condon and other approximations frequently used in theoretical treatments of such systeWe
compare the Franck-Condon approximation with the exact solution for the ultrafast nonadiabatic dynamics of a
four-dimensional model of pyrazine.
755
Accelerating calculations of ultrafast time-resolved electronic spectra with various high
order split-operator algorithms
Marius Wehrle, Jiri Vanicek, Laboratory of theoretical physical chemistry, EPFL SB SCGC-GE, 1015 Lausanne
Calculations of ultrafast time-resolved electronic spectra require performing the quantum dynamics, i.e., solving
the time-dependent Schrödinger equation. We explore how these demanding calculations can be accelerated
by increasing the time step required for convergence of exact quantum simulations without sacrificing the accuracy [1]. For this purpose, we compared various split-operator methods of the 1st to 4th order. Besides the
usual 4th order algorithm with real coefficients we also considered an algorithm with complex coefficients and an
algorithm using the gradient of the potential. Our results show that while the last two methods require fewer Fast
Fourier Transforms at each step, the complex method diverges for dense spatial grids and the gradient method's
order decreases to the 2nd order for sparse grids [1].
[1] M. Wehrle, M. Sulc, and J. Vanicek, Chimia 65, 334 (2011)
756
High-harmonic spectroscopy of isoelectronic molecules:
electronic structure and multielectron effects
Alisa Rupenyan-Vasileva, Stefan Vlajkovic, Peter Kraus, Hans Jakob Wörner
LPC, ETH Zürich, Wolfgang-Pauli-Strasse 10, 8093 Zürich
High-harmonic spectra of aligned molecules carry information about the electronic structure. A pronounced
minimum in the spectrum of aligned CO2 molecules has been previously assigned to structure, dynamics or the
interplay of both. We combine high-harmonic spectroscopy with theoretical modeling of aligned CO2 and an
isoelectronic molecule, N2O to understand which of these interpretations is most appropriate. We find that the
position of the spectral minimum differs in N2O and CO2. Although the bond lengths of CO2 and N2O are almost
equal, the nodal plane in the highest-occupied molecular orbital (HOMO) of N2O is shifted towards the O-atom
compared to CO2, which is shown to rationalize the difference in the observed positions of the minima. Thus, the
different positions of the minima are related to the electronic structure of the molecules and containinformation
about the location of the nodal plane of their HOMO.
123
757
Actively Stabilized Attosecond Interferometer
Martin Huppert, Hans Jakob Wörner, LPC, ETH Zürich, Wolfgang-Pauli-Strasse 10, 8093 Zürich
Our setup generates a controlled attosecond delay between two ultrashort infrared or ultraviolet and extreme
ultraviolet pulses and represents a versatile tool to realize pump-probe measurements of electronic dynamics in
polyatomic molecules. Our innovative setup will be combined with a currently operating velocity-map-imaging
(VMI) spectrometer or, alternatively, a planned photoelectron time-of-flight spectrometer for gaseous and liquid
samples. Molecular systems with chemical relevance will be studied. To reach long term stability of the pulse
delay with accuracy in the attosecond range, the delay line has to be actively stabilized. We are using a sophisticated stabilization scheme to stabilize at arbitrary delays from zero to about one picosecond. The feedback
loop is realized fully digitally in a real-time computer system, which allows loop rates in the kilohertz range. In
addition, a white-light interferometer gives information about the absolute path length difference between the
two interfering beams.
758
Ultrafast time-resolved photoelectron spectroscopy of solvated systems
Inga Jordan, Hans Jakob Wörner, Matthew Brown, Jeroen van Bockhoven
LPC, ETH Zürich, Wolfgang-Pauli-Strasse 10, 8093 Zürich
Our ultrafast pump-probe experiment combines the liquid microjet technique with a high harmonic photoionization source and an attosecond interferometer. We intend to transfer the information content of photoelectron
spectroscopic studies from the determination of the static electronic structure of solids to the observation of
ultrafast electronic dynamics of solvated systems in real time. A thin liquid filament (~20 μm) is injected into the
interaction chamber through a nozzle and travels several millimeters as a laminar flow before being caught by a
catcher and fed into in a recycling mechanism. The laser radiation hits the liquid filament exactly in front of the
orifice of a microskimmer, which operates as differential pumping stage and also defines the entrance to the
electron spectrometer. The interaction chamber, the electron spectrometer and the cooling system required for
reasonable pressures are currently being constructed.
759
Versatile velocity-map-imaging spectrometer for strong-field and attosecond experiments
Samuel Walt, Hans Jakob Wörner, LPC, ETH Zürich, Wolfgang-Pauli-Strasse 10, 8093 Zürich
We present an innovative velocity-map-imaging spectrometer that allows a continuous tuning of the particle
density and the sample temperature. This new design allows us to optimize the experimental conditions for a
range of experiments using intense infrared laser pulses and/or attosecond pulses. We will use laser-induced
electron tunneling and rescattering for probing nuclear rearrangements in photochemical reactions and imaging
proton dynamics on an attosecond timescale. With techniques of impulsive alignment and orientation, molecules will be fixed in space. As a result, the laser-driven electron rescattering process is defined with respect
to the molecular frame. Photochemical reactions will be investigated using these new strong-field techniques. A
pump pulse will be used to photoexcite the molecule under study and the subsequent probe pulse will retrieve
the structural information.
760
Versatile Non Collinear Four-Wave Mixing Set-Up Fully Based on Femtosecond Pulse
Shaping for Coherent Electronic Spectroscopy
Franziska Frei, Andrea Cannizzo, Thomas Feurer
Institute for Applied Physics, University of Bern, Sidlerstr. 5, 3012 Bern
Femtosecond pulse shaping is a versatile tool in many different fields. Nowadays, arbitrary pulse shaping is used
for optical pulse compression, for coherent control, in nonlinear spectroscopy or for biological imaging applications, etc. Moving to coherent spectroscopy, the unique capability of pulse shaping methodology to provide
phase stable pulses is well accepted and several schemes have been proposed to implement four wave mixing
(4WM) set-ups based on pulse-shaping. Another unique advantage is the capability to enhance specific signals
and suppressing background scattering by phase-cycling the three interacting pulses and the local oscillator.
Non collinear 4WM set-ups benefit from a good background rejection at the cost of no independently shaped
pulses. Very recently non collinear 4WM spectrometers based on pump probe schemes have been implemented
where the pump pulse is replaced by a pair of pulses generated via pulse shaping. This approach provides a very
robust set-up for multidimensional spectroscopy and it is relatively easy to be implemented. Unfortunately, with
the third pulse not phase locked to the other two beams, the extent of background rejection with phase-cycling
is rather limited. To fully benefit from a non collinear geometry and a pulse generation entirely based on pulse
shaping, we recently implemented a non collinear 4WM set-up fully based on femtosecond pulse shaping, accordingly to a scheme previously developed by Nelson and collaborators. All the four beams are simultaneously
shaped in the same pulse-shaper employing a two dimensional liquid crystal spatial light modulator. In this way
on the one hand all the pulses are inherently phase-locked on the other hand each pulse can be independently
124
shaped. This allows for coherent VIS and IR spectroscopy on molecular systems with a full and independent
control of the time properties of each of the four beaIn particular we have no movable mechanical parts in the
set-up with an uncertainty on the relative time delay of 0.150 fs, and we can optimize the shaper in order to
have identical pulses at the sample position. In this contribution, we will show the main characteristics of the
experimental setup and demonstrate its performance with several examples from different molecular systems.
761
Field Enhancement in THz nano-structures
Fabian Brunner, Salvatore Bagiante, Florian Enderli, Justyna Fabianska, Thomas Feurer
Institute for Applied Physics, University of Bern, Sidlerstr. 5, 3012 Bern
The last decades have seen significant progress in the development of THz technologies in a variety of research
fields and applications such as chemical recognition, material inspection, or security control. One of the major
and yet unmet limitations is that, compared to the optical regime, the pulse energies supplied by current THz
sources are still rather limited preventing the advent of high fidelity THz nonlinear spectroscopy. The highest
average THz power levels currently available come from large-scale electron accelerators and table-top laser
sources using for example large area photo-conductive switches, frequency mixing in laser-generated plasmas,
or optical rectification in nonlinear crystals. To extend THz experiments into the nonlinear regime, i.e. for electric
or magnetic switching applications or nonlinear spectroscopy, the quantity to be optimized is essentially the
electric field strength, since a nonlinear process scales with a power of the electric field. The field strength is
linked to the pulse energy, the pulse duration, and the beam area. The available pulse energy is naturally limited
by the THz system at hand and the pulses are typically already single-cycle, so that one cannot increase the
field strength by adjusting these parameters. The beam area can be minimized through tight focusing typically
with parabolic mirrors. However, the diffraction limit imposes a lower boundary thereby restricting the obtainable
field strength. Recently, several reports have shown that the limitations due to diffraction limited focusing can be
overcome by using metallic nano-structures that collect the incident radiation and focus it in a sub-wavelength
volume leading to strong field enhancement. This concept was introduced by Seo and co-workers who have
reported that a single nano-slit in a thin gold film may act effectively as a nano-capacitor. Here, we introduce
different geometries in order to further increase the obtainable field enhancement. The structures are resonant
in the THz regime and feature extremely small gaps. Due to the resonant behavior the resulting field strengths in
the gap region are significantly higher than those for the non-resonant nano-slits. Giant enhancement factors of
tens of thousands are obtained. Enhancing the field strength is one side of the medal; the other is to maximize
the integrated nonlinear response. To that end the overall size of the volume comprising the high field strengths
has to be maximized. We therefore investigate how the nonlinear response can be maximized by adjusting the
structural parameters.
762
Femtosecond surface second harmonic generation microscopy
to probe adsorbed layers at interfaces
Astrid Olaya 1, Delphine Schaming 1, Micheal Scanlon 1, Pierre-François Brevet 2, Hubert H. Girault 1
1
Laboratoire d'Electrochimie Physique et Analytique, EPFL,1015 Lausanne
2
Lab. de Spectrométrie Ionique et Moléculaire, Univ. Claude Bernard Lyon 1, FR-69622 Villeurbanne Cedex
A new geometrical configuration based on a confocal multiphoton microscopy has been used to investigate by
SHG the surface of suspended small liquid drops. The advantage of this approach is the possibility to measure
non-linear properties at a resolved spatial scale using microscopy and with a much greater sensitivity due to increased intensities. Also, it offers the possibility to compare on a single experiment the coherent SSHG intensity
collected from the surface and the non-coherent SHG intensity arising from the bulk volume (HRS). Aqueous
solutions of gold nanoparticles have been studied to assess the performances of this experimental setup and
to monitor the formation of a gold mirror at the air-water interface. A model has also been developed to analyze
the data and extract the main optical parameters.
763 Interfacial Self-Assembly of Aqueous Cationic Porphyrins for Reducing Oxygen to Water
Astrid Olaya 1, Pierre-François Brevet 2, Hubert H. Girault 1
Laboratoire d'Electrochimie Physique et Analytique,EPFL, 1015 Lausanne
2
Lab. de Spectrométrie Ionique et Moléculaire, Univ. Claude Bernard Lyon 1, FR-69622 Villeurbanne Cedex
1
Liquid-liquid interfaces can be electrically polarised and a key unresolved question is to know if the kinetics of
electron transfer reactions between an electron donor in one phase and an electron acceptor in the other phase
is dependent on the local electric potential difference. To answer this fundamental question, a time resolved surface second harmonic setup similar to that recently proposed by Vauthey's group in Geneva is being assembled
to study interfacial photo-induced electron transfer reactions. So far, we have used surface second harmonic
125
generation to monitor the self-assembly of porphyrin molecules at the interface. Surprisingly, the self-assembled
molecular architectures have shown to be excellent catalysts for oxygen reduction. The next step is to measure
the kinetics of their photo-induced reactivity.
764
Time-resolved X-ray absorption studies on charge carrier dynamics
in aqueous TiO2 nanoparticles
Mercedes H. Rittmann-Frank, C. J. Milne, J. Rittmann, M. Reinhard, M. Silatani, T. Rossi, M. Chergui
EPFL ISIC-FSB, Laboratoire de Spectroscopie Ultrarapide, 1015 Lausanne
Significant effort has been put into understanding electron/hole dynamics in electronically excited TiO2 nanoparticles (NPs) and how the kinetics of the charge carriers are affected by changes in the electronic and structural
properties of such systems. This is necessary in order to understand the underlying mechanisms occurring in
photocatalysis and in dye sensitized solar cells (DSSC), for which TiO2 is the main material. Here we present a
new approach investigating the charge carrier dynamics in nanoparticles using time-resolved X-ray absorption
spectroscopy (XAS), which show how the electron dynamics affect not only the electronic but also the structural
properties of colloidal TiO2 NPs.
We use our recently developed high repetition-rate pump-probe XAS setup [1] at the Swiss Light Source (Paul
Scherrer Institut), pumping with a 260 kHz ps laser pulse at 355 nm and probing the spectral changes at the Ti
K-edge (4.9 keV). Figure 1 shows the static Ti K-edge spectrum (black) and the difference spectrum (excited
minus unexcited sample transmission, in blue) recorded at 100 ps time delay. Significant changes are observed
in the pre-edge region below 4.981 keV, which are related to bound-bound electronic transitions and contain
information on the conduction band states of the semiconductor NP. There is acontribution due to the shift of
the edge to lower energies (see inset), pointing to a change from Ti4+ to Ti3+, and indicating the presence of a
trapped electron at the Ti site. Differences in the EXAFS region indicate a change of structure around the Ti
atom. Under the laser fluences used here, this points to a transition from anatase towards a more disordered
local geometry. These results underscore the correlation between electronic relaxation and the electronic and
geometrical structural changes in the NPs.
[1] F. Lima et al., Rev. Sci. Instrum. 82( 2011) 063111
765
Probing the structural dynamics of hemoproteins in solution
by time-resolved x-ray absorption spectroscopy
Masha Silatani 1, F. A. Lima 1, C. J. Milne 1, D. C. V. Amarasinghe 1, R. M. van der Veen 1, 2, M. Reinhard 1,
M. H. Rittman-Frank 1, D. Grolimund 2, C. Borca 2, R. Abela 3, F. van Mourik 1, M. Chergui 1
1
Ecole Polytechnique Fédérale de Lausanne, LSU, Lausanne
2
SLS, Paul Scherrer Institut, Villigen, 3 SwissFEL, Paul Scherrer Institut, Villigen
Myoglobin (Mb) is one of the best-characterized metalloproteins, its main function being the facilitation of oxygen diffusion in muscle cells.[1] Understanding the mechanism by which Mb discriminates amongst the various
small molecules, or ligands, that can bind to its active center has been the subject of many studies over the
years. Despite the wealth of information available from numerous experimental and theoretical studies, a microscopic description of ligand detachment and rebinding remains elusive, and fundamental questions are still to
be answered. X-ray absorption spectroscopy (XAS) is an ideal technique for determining local geometric and
electronic structure in disordered systems, such as proteins in solution. The ground-state XAS of Mb with six different ligands in the ferric (metMb, MbCN) and ferrous (deoxyMb, MbCO, MbNO, MbO2) oxidation states of the
iron atom in the protein were measured at the Fe K-edge (7 keV) under physiological conditions such as low pH
and low contentrations. Using continuously flowing liquid samples, which allows for continuous exchange of the
molecules in the x-ray focus, dramatically lowers the accumulated x-ray dose for each individual protein, thus
avoiding x-ray induced damage effects. The full multiple scattering formalism is applied using the MXAN code
[2] to calculate the near-edge XAS and interpret the features present on the basis of the local density-of-states.
Structural fits to the experimental spectra were performed by changing the geometric parameters of the protein
around the absorbing iron atom. The assignment of the transitions causing the pre-edge features of the spectra
of Mb, often neglected in the interpretation of the XAS, were investigated in detail by means of time-dependent
density functional theory (TDDFT). Our results are in good agreement with the structures determined by x-ray
crystallography, showing the power of the technique in determining local structures of proteins in physiological
conditions. The TDDFT analysis indicates that a simple atomic picture of a Fe 1s to 3d transition is not sufficient
to explain the presence and shape of these pre-edge features and show their remarkable sensitivity to the local
protein geometry and the chemical nature of the ligand. Our analysis of the electronic and geometrical structureof the different forms of Mb is believed to be the first conducted on this series of proteins in physiological
conditions and represents the first step towards a better understanding of our recent time-resolved XAS measurements [3] on the MbCO and MbNO forms of the protein.
126
[1] L. J. Kagen. Myoglobin: Biochemical, Physiological, and Clinical Aspects. Columbia University Press, 1973.
[2] M. Benfatto et al. MXAN: A new software procedure to perform geometrical fitting of experimental XANES spectra. J. Synchr.
Rad., 8(2):267–269, 2001; M Benfatto et al. The MXAN procedure: A new method for analysing the XANES spectra of metalloproteins to obtain structural quantitative information. J. Synchr. Rad., 10(1):51–57, 2003.
[3] F. Lima et al. A high-repetition rate scheme for synchrotron-based picosecond laser pump/x-ray probe experiments on chemical
and biological systems in solution. Rev. Sci. Inst., 82(6):063111, 2011.
766
THz-IR Mode Coupling in Chemisorbed CO on Pt
Anastasija Ichsanow 1, Clemens Ruchert 1, Hans C. Sigg 1, Christoph P. Hauri 1, Jeroen A. van Bokhoven 1, 2,
Rafael Abela 1, Bruce D. Patterson 1
1
Paul Scherrer Institute, 5232 Villigen
2
ICB, ETH- Zürich, Wolfgang-Pauli-Strasse 10, 8093 Zürich
In anticipation of pump-probe experiments at the SwissFEL X-ray Free Electron Laser, we investigate the possibility of initiating surface catalytic reactions with energetic terahertz pulses. The system we have chosen,
chemisorbed CO on nano-particulate Pt, has a „hindered translation“ vibration mode at approximately 2.5 THz
and a C-O bond stretch mode, corresponding to an IR wavelength of 4.8 µm. On surfaces of transition metals to the right of column 6 in the periodic table, CO adsorbs in molecular form, while on metals to the left, it
spontaneously undergoes "adsorptive dissociation". We resonantly excite the hindered translation of CO on
Pt with 800 kV/cm THz pulses, obtained from difference frequency generation in the organic non-linear crystal
DAST, and we use time-resolved IR absorption to detect coupling to the C-O stretch mode, as a pre-cursor to
THz-induced dissociation.
767
UV Two-Dimensional Spectroscopy for Biological Systems
Gerald Auböck, Cristina Consani, Frank van Mourik, Majed Chergui
EPFL, Laboratory of Ultrafast Spectroscopy, 1015 Lausanne
We recently developed a novel approach to two-dimensional (2D) femtosecond spectroscopy in the ultraviolet
(UV) [1], which combines a broad band UV probe pulse with a widely frequency tuneable pump pulse. This allows us to acquire 2D UV spectra with an unprecedented window of more than 80 nm and a temporal resolution
of 120 fs as demonstrated by measurements on the UV dye 2,5-Diphenyloxazol dissolved in Cyclohexane. Since
the absorption bands of most biologically relevant species (eg. aromatic amino acids in proteins, DNA bases)
extend over several tens of nm, this large bandwidth allows addressing for the first time a wide variety of questions concerning protein dynamics.
To demonstrate the feasibility of such studies we will present results on ferric myoglobins (met and cyano)
under physiological conditions. These systems show overlapping absorption bands of the heam and the two
tryptophan residues in the UV (<300 nm). We demonstrate that 2D spectra taken at a sufficiently large number
of time delays allow a clean separation of these highly congested spectra by a global analysis of the 2D spectra.
In addition to the well-known Förster resonant energy transfer from the two tryptophan residues to the haem,
we also observe a competing Trp to haem electron transfer from the Trp14 residue, located close to the haem
group. Combination of the UV 2D data with broadband visible transient absorption experiments also enables
us to follow the full haem photo-cycle which was, in spite of many previous studies, still not fully characterized.
[1] G. Auböck, C. Consani, F. van Mourik and M. Chergui, Optics Letters, accepted (2012)
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