Bulletin of the American Physical Society
46th Annual Meeting of the APS Division of Atomic, Molecular and Optical Physics
Volume 60, Number 7
Monday–Friday, June 8–12, 2015; Columbus, Ohio
Session G4: Invited Session: Chemical and Interfacial Dynamics |
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Chair: Marcos Dantus, Michigan State University Room: Union DE |
Wednesday, June 10, 2015 8:00AM - 8:30AM |
G4.00001: Attosecond Molecular Dynamics Invited Speaker: Fernando Martin The development of attosecond laser pulses allows one to probe the inner working of atoms, molecules and surfaces on the timescale of the electronic response. In molecules, attosecond pump-probe spectroscopy enables investigations of the prompt charge redistribution and localization that accompany photo-excitation processes, where a molecule is lifted from the ground Born-Oppenheimer potential energy surface to one or more excited surfaces, and where subsequent photochemistry evolves on femto- and attosecond timescales. In this talk I will present a few theoretical examples of realistic molecular attosecond pump-probe experiments in which simple molecules are ionized with a single attosecond pulse (or a train of attosecond pulses) and are subsequently probed by one or several infrared or xuv few-cycle pulses. The evolution of the electronic and nuclear densities in the photo-excited molecule or remaining molecular ions is calculated with attosecond time-resolution and is visualized by varying the delay between the pump and probe pulses. The results of these calculations [1-7] allow us to explain several experimental observations as well as to guide future experimental efforts to uncover ultrafast electron and nuclear dynamics in molecules. \\[4pt] [1] G. Sansone et al, Nature 465, 763 (2010).\\[0pt] [2] S. E. Canton et al, Proc. Natl. Acad. Sci. 108, 7302 (2011)\\[0pt] [3] A. Gonz\'{a}lez-Castrillo et al, Phys. Rev. Lett. 108, 063009 (2012)\\[0pt] [4] A. Palacios et al, Proc. Natl. Acad. Sci. 111, 3973 (2014).\\[0pt] [5] F. Calegari et al, Science 346, 336 (2014)\\[0pt] [6] C. Ott et al, Nature 516, 374 (2014)\\[0pt] [7] P. Ranitovic et al, Proc. Natl. Acad. Sci. 111, 912 (2014) [Preview Abstract] |
Wednesday, June 10, 2015 8:30AM - 9:00AM |
G4.00002: Progress in coherence and dynamics of x-ray and inner-shell processes Invited Speaker: Stephen Southworth X-ray and inner-shell processes are being investigated with new experimental and theoretical tools. X-ray free-electron lasers (XFELs) can generate one or two intense, ultrafast x-ray pulses that produce inner-shell holes and probe their decays in atoms, molecules, clusters and nanoparticles. Seeding techniques generate XFEL pulses with narrow bandwidths and high temporal coherence that enhance opportunities for exploiting quantum optics methods in the x-ray regime. Optical lasers combined with x-rays can control populations of core-excited states, exploit sidebands on resonant Auger transitions, and explore interatomic charge transfer [1,2]. Theoretical simulations can model the complex ionization pathways initiated in a many-electron atom by an XFEL pulse [3]. Those topics are reviewed along with results of pump-probe experiments using two XFEL pulses to produce inner-shell holes and probe the electronic decays and fragmentation of molecular ions. \\[4pt] [1] A. {Pic\'on} \textit{et al.}, Phys. Rev. A {\bf 87}, 013432 (2013); New J. Phys. {\bf 15}, 083057 (2013).\\[0pt] [2] B. Erk \textit{et al.}, Science {\bf 345}, 288 (2014).\\[0pt] [3] P. J. Ho \textit{et al.}, Phys. Rev. Lett. {\bf 113}, 253001 (2014). [Preview Abstract] |
Wednesday, June 10, 2015 9:00AM - 9:30AM |
G4.00003: Probing interfacial electron dynamics with time-resolved X-ray spectroscopy Invited Speaker: Stefan Neppl Time-resolved core-level spectroscopy techniques using laser pulses to initiate and short X-ray pulses to probe photo-induced processes have the potential to provide electronic state- and atomic site-specific insight into fundamental electron dynamics at complex interfaces. We describe the implementation of femto- and picosecond time-resolved photoelectron spectroscopy at the Linac Coherent Light Source (LCLS) and at the Advanced Light Source (ALS) in order to follow light-driven electron dynamics at dye-semiconductor interfaces on femto- to nanosecond timescales, and from the perspective of individual atomic sites. A distinct transient binding-energy shift of the Ru3d photoemission lines originating from the metal centers of N3 dye-molecules adsorbed on nanoporous ZnO is observed 500 fs after resonant HOMO-LUMO excitation with a visible laser pulse [1]. This dynamical chemical shift is accompanied by a characteristic surface photo-voltage response of the semiconductor substrate [2]. The two phenomena and their correlation will be discussed in the context of electronic bottlenecks for efficient interfacial charge-transfer and possible charge recombination and relaxation pathways leading to the neutralization of the transiently oxidized dye following ultrafast electron injection. First steps towards \textit{in operando} time-resolved X-ray absorption spectroscopy techniques to monitor interfacial chemical dynamics will be presented. \\[4pt] [1] Siefermann \textit{et al.} J. Chem. Phys. Lett \textbf{5}, 2753 (2014)\\[0pt] [2] Neppl \textit{et al.} Faraday Discuss. \textbf{171}, 219 (2014) [Preview Abstract] |
Wednesday, June 10, 2015 9:30AM - 10:00AM |
G4.00004: Ultrafast Structural Dynamics by X-Ray Diffraction and Structural Spectroscopy Invited Speaker: Peter M. Weber The ability to observe molecular reactions in real time is expected to aid the exploration of new reaction mechanisms, the development of catalysts, the understanding of biomolecular processes and the control of chemical reactions and material properties on a molecular level. To reach this goal, we have developed a gas-phase x-ray diffraction experiment that uses the ultrashort x-ray pulses from the Linac Coherent Light Source (LCLS) to capture atomic motions within molecules in a dilute gas ($<$ 5 Torr). The delay time dependence of the gas x-ray diffraction pattern is measured in a pump-probe scheme with 267 nm excitation laser and 8.3 keV X-ray probe pulses. Optical excitation prepares 1,3-cyclohexadiene on the excited 1B surface, from where it accelerates past a conical intersection down the 2A potential energy surfaces before opening the ring structure on a 140 fs time scale. A ``molecular movie'' of the observed dynamics is constructed by comparing ab initio quantum molecular dynamics simulations with the experimental diffraction signal to derive weighted trajectories that provide a good representation of the structural dynamics, with the weighted ensemble of trajectories corresponding to the nuclear flux during the chemical reaction. The x-ray structural data thus provide reaction pathways for which ionization energies can be calculated at each step. We use ultrafast time-resolved multiphoton - ionization photoelectron spectroscopy to measure the travel time required for the molecule to reach certain resonance windows to Rydberg states. By so combining the results from the ultrafast x-ray diffraction with observations from ultrafast (structural) spectroscopy, it appears that we can make significant progress towards the ultimate goal: a comprehensive understanding of the spatially resolved photochemical reaction dynamics. [Preview Abstract] |
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