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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. 21 114 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. 115 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 116 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. 117 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. 118 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. 120 744 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. 121 748 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) 127