Bulletin of the American Physical Society
2007 APS March Meeting
Volume 52, Number 1
Monday–Friday, March 5–9, 2007; Denver, Colorado
Session V19: Focus Session: Ultrafast Dynamics using X-rays and Electrons II |
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Sponsoring Units: DCP Chair: Christoph Rose-Petruck, Brown University Room: Colorado Convention Center 104 |
Thursday, March 8, 2007 11:15AM - 11:51AM |
V19.00001: Picosecond X-ray absorption spectroscopy of light-induced processes in liquids Invited Speaker: The importance of capturing the dynamics of molecular motion in real-time is crucial for our understanding of physical, chemical and biological phenomena. In addition, structural changes in molecular systems stem from ultrafast electronic structure changes, which change the field of forces within a molecule, and between a molecule and its environment. Therefore, observing both electronic and structural changes in a given system provides deeper insight into its dynamics. Ultrafast optical spectroscopy does not deliver molecular structure. Ideal structural tools in this respect are X-rays, via methods such as diffraction or spectroscopy. Here, we will present our recent results on the probing of structural changes in electronically excited solvated species, using picosecond X-ray absorption spectroscopy, in a laser pump/X-ray probe configuration. We will demonstrate the power of this new approach on three different types of processes: intramolecular electron transfer, ultrafast molecular magnetism and solvation dynamics around an atomic ion, and discuss future extensions to biological systems. We will also discuss future experiments with femtosecond time resolution at synchrotrons and 4$^{th}$ generation light sources. [Preview Abstract] |
Thursday, March 8, 2007 11:51AM - 12:27PM |
V19.00002: Ultrafast Structural Dynamics of Photoactive Metal Complexes in Solar Energy Conversion. Invited Speaker: The photoexcited states of metal complexes are precursors for many important photochemical processes in solution phase. Using laser-initiated time-resolved x-ray absorption spectroscopy (LITR-XAS), transient metal oxidation states, coordination geometry, and atomic rearrangements that closely reflect photochemical processes can be probed, which complements with ultrafast optical laser spectroscopic studies for kinetics and coherence among different excited states as well as intra- and intermolecular energy/charge transfer processes associated with solar energy conversion. We have studied by LITR-XAS combined with transient absorption spectroscopy excited state structures, such as metalloporphyrins and platinum(II) complexes, in solution. Direct evidence of photoinduced redox reactions and coordination geometry changes have been observed. These experimental studies are combined with time-dependent density functional theory (TDDFT) calculations to rationalize the excited state structural nuclear changes with electronic configuration changes that may be responsible for the reactivity of the molecules. These studies will have a great impact in fundamental understanding of solar fuel production. [Preview Abstract] |
Thursday, March 8, 2007 12:27PM - 1:03PM |
V19.00003: Watching proteins function with 150-ps time-resolved X-ray crystallography Invited Speaker: We have used time-resolved Laue crystallography to characterize ligand migration pathways and dynamics in wild-type and several mutant forms of myoglobin (Mb), a ligand-binding heme protein found in muscle tissue. In these pump-probe experiments, which were conducted on the ID09B time-resolved beamline at the European Synchrotron and Radiation Facility, a laser pulse photodissociates CO from an MbCO crystal and a suitably delayed X-ray pulse probes its structure via Laue diffraction. Single-site mutations in the vicinity of the heme pocket docking site were found to have a dramatic effect on ligand migration. To visualize this process, time-resolved electron density maps were stitched together into movies that unveil with $<$2-{\AA} spatial resolution and 150-ps time-resolution the correlated protein motions that accompany and/or mediate ligand migration. These studies help to illustrate at an atomic level relationships between protein structure, dynamics, and function. [Preview Abstract] |
Thursday, March 8, 2007 1:03PM - 1:15PM |
V19.00004: The Laser-Assisted Photoelectric Effect on Surfaces Luis Miaja-Avila, Guido Saathoff, Chifong Lei, Margaret Murnane, Henry Kapteyn, Martin Aeschlimann, John Gland The laser-assisted photoelectric effect (LAPE) in atoms is widely used for the characterization of ultrashort EUV pulses and for femtosecond-to-attosecond spectroscopy. We recently observed the equivalent process in the original manifestation of the photoelectric effect i.e. photoemission from surfaces [PRL \textbf{97, }113604 (2006)]. In our experiment, ultrafast 800nm pulses are split into probe and pump beams. The probe beam is upconverted into the EUV at 30nm using high harmonic frequency conversion. The 800nm pump beam is spatially and temporally overlapped with the EUV beam on a Pt(111) sample. A time-of-flight detector measures the kinetic energy of the photoemitted electrons. In the presence of the pump beam, these electrons can either absorb or emit an IR photon, leading to sidebands in the EUV photoelectron spectrum. These sidebands are visible as modulations near the Fermi edge. Surface LAPE will extend EUV pulse measurements to higher photon energies. It also has the potential to study ultrafast, femtosecond-to-attosecond time-scale processes in solids and in surface-adsorbate systems, where complex, correlated, electron dynamics are expected. [Preview Abstract] |
Thursday, March 8, 2007 1:15PM - 1:27PM |
V19.00005: Dynamics of small polaron formation in epitaxial pentacene films Matthias Muntwiler, William Tisdale, Ehua Fan, Chad Lindstrom, Xiaoyang Zhu Using time-resolved two-photon photoemission (TR-2PPE) we observe the formation of a small polaron from electrons injected into the LUMO band of thin epitaxial pentacene films. Such observation is made directly in the energy and time domains by analysing the photoelectron after excitation by pump and delayed probe pulses from a femtosecond laser system. The LUMO level of pentacene is observed in a charge transfer process that originates from an initial state of the substrate and as such does not involve exciton formation. Due to interaction with the nuclear lattice, the energy level of the LUMO-derived polaron state relaxes by about 200 meV towards lower energy over a time interval of several hundred femtoseconds. Small polarons account for one possible mechanism of charge trapping in organic semiconductors. In our experiments, pentacene films are grown in a bulk-like structure (standing phase) on a thin film Bi(111) substrate. [Preview Abstract] |
Thursday, March 8, 2007 1:27PM - 1:39PM |
V19.00006: Dynamics on nanointerfaces investigated by ultrafast electron nano-crystallography Chong-Yu Ruan, Yoshie Murooka, Ramani Kalyan Raman, Ryan Murdick The correlation between the material structures and the size-dependent properties is a fundamental problem in nanoscience. Through the development of ultrafast electron nano-crystallography and spectroscopy techniques, we have looked at some important mechanisms pertaining to the nanometer scales. To accentuate the structurally correlated transformations from bulk to the molecular length scale, we size-select and deposit nanoparticles (Au, Ag) on well characterized interfaces. Using femtosecond optical pulse as the pump and femtosecond electron pulse as the probe, the electronic and temperature driven transformations of nanostructures and phases are examined at calibrated levels on the energy landscape with atomistic spatialtemporal resolutions ($\le $ 10$^{-12}$ sec, $\le $ 0.01 {\AA}). We observed the ultrafast nonthermal phase transformations of nanoparticles with transient full-scale radial distribution function accurately determined. The coherent motions of atoms driven by nonthermal energy transfer persist into the liquid phase. But the recrystallization process is more thermal-like with interesting reconstruction of lattices from the melt, nonreciprocal to that of melting. We also examined the ultrafast molecular structural responses to charge transfer that exhibits a dynamical phase transition going from conducting to insulating phases. [Preview Abstract] |
Thursday, March 8, 2007 1:39PM - 1:51PM |
V19.00007: Multiple Reference Soft X-ray Fourier Transform Holography W. Schlotter, K. Chen, R. Rick, A. Scherz, J. Stohr, J. Luning We demonstrate multiple reference Fourier transform holography with soft x-rays. This technique results in enhanced image quality without increased exposure to the sample. There are two categories of experiments where this is particularly relevant: imaging with limited intensity sources and imaging radiation sensitive samples. In Fourier transform holography a unique image of the sample object is reconstructed for each reference source that is used to record the hologram. Thus with multiple reference sources, multiple images are reconstructed with the same radiation exposure to the sample necessary for a single image. When the multiple images are formed by effectively identical reference sources the images can be averaged to enhance image quality. X-ray free electron lasers are an example of sources where single shot images must be captured with one ultrafast x-ray pulse. Since the number of photons in each pulse, incident on a nanoscale sample, is finite, high efficiency imaging is essential. To mimic a finite illumination scenario we have successfully imaged a nanoscale test object by detecting fewer than 2500 soft x-ray photons. [Preview Abstract] |
Thursday, March 8, 2007 1:51PM - 2:03PM |
V19.00008: X-ray Resonant Photoemission in organic thin films. Alberto Morgante, Dean Cvetko, Albano Cossaro, Luca Floreano, Gregor Bavdek Resonant photoemission (RESPES) allows to investigate the charge transfer processes in thin films at the femtosecond time scale and it has been recently applied to thin organic films on inorganic substrates in order to obtain information related to the carrier injection at the interface, a process of great importance for organic electronic applications. High resolution RESPES on monolayer and multilayer organic films on semiconductor and metal substrates at C K-edge will be presented. It will be shown that in the monolayer range RESPES makes possible to clearly identify very weak molecular valence band structures which can't be distinguished from substrate ones in normal photoemission. Moreover the charge transfer time information will be deduced from the spectroscopic data by the comparison of monolayer and multilayer RESPES spectra. It will be also demonstrated that, due to the localized nature of the resonant process, RESPES gives a clear spatial correlation between filled and empty states and that this effect should be carefully taken into account in the analysis of the resonant spectra for the charge transfer time determination. [Preview Abstract] |
Thursday, March 8, 2007 2:03PM - 2:15PM |
V19.00009: Atomic Scale Force Spectroscopy Suggests Low Reorganization Entropy For Electron Transfer J.T. Sage, A. Barabanschikov, B. Barbiellini, E.D. Coulter, D.M. Kurtz, E.E. Alp, W. Sturhahn Molecular mechanics simulations of the electron transfer protein rubredoxin suggest significant delocalization of active site vibrations, but experimental vibrational data have been successfully modelled using empirical models that exclude distant atoms. We address this question using nuclear resonance vibrational spectroscopy (NRVS), an emerging synchrotron-based technique that yields the complete vibrational spectrum of $^{57}$Fe in rubredoxin, coupled with quantum chemical vibrational predictions. The measured vibrational density of states reveals that oxidation strengthens the Fe-S bonds linking iron to the polypeptide, as expected from the reported decrease in Fe-S bond lengths. Moreover, comparison with Raman isotope shift measurements provides direct experimental evidence that the Fe-S vibrations remain localized at the active site. Vibrational predictions based on DFT calculations reproduce the observed vibrational data and confirm the localized nature of the Fe-S vibrations, although lower frequency vibrations mix significantly with the polypeptide. The Fe-S vibrations are an important component of the electron transfer reaction coordinate, and we suggest that vibrational localization may facilitate electron transfer by minimizing the reorganization entropy of the reaction. [Preview Abstract] |
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