Bulletin of the American Physical Society
APS March Meeting 2018
Volume 63, Number 1
Monday–Friday, March 5–9, 2018; Los Angeles, California
Session B25: Ultrafast Laser Techniques for Molecular Photochemistry and PhotophysicsInvited
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Sponsoring Units: DLS Chair: Anne Kelley, University of California - Merced Room: LACC 403B |
Monday, March 5, 2018 11:15AM - 11:51AM |
B25.00001: Light, Molecules, Action: Using Ultrafast Optical and X-ray Spectroscopy to Probe Excited State Dynamics in Photoactive Molecules. Invited Speaker: Roseanne Sension Light provides a versatile energy source capable of precise manipulation of material systems on size scales ranging from molecular to macroscopic. Photochemistry provides the means for transforming light energy from photon to process via movement of charge, a change in shape, a change in size, or the cleavage of a bond. Put simply, photochemistry produces action. In the work to be presented here ultrafast UV-Visible and X-ray absorption spectroscopies have been applied to the study of cobalamin (vitamin B12) based compounds. Optical measurements provide precise characterization of spectroscopic signatures of the intermediate species on the excited state surface responsible for photochemistry. In contrast time-resolved XANES at the Co K-edge is sensitive to the structural changes that accompany these transformations. Polarized tr-XANES measurements allow identification of concurrent and sequential structural dynamics on the femtosecond and picosecond time scales. The results of recent polarized ultrafast XANES measurements at LCLS on two photoactive B12 compounds will be presented. |
Monday, March 5, 2018 11:51AM - 12:27PM |
B25.00002: What do Ultrafast Photoelectron Spectroscopy and Temperature-Dependent Transient Absorption Experiments Tell Us About the Structure of the Hydrated Electron? Invited Speaker: Benjamin J. Schwartz Despite intense investigation, it is still unclear whether the structure of an excess electron in liquid water is best thought of as something similar to a halide ion, where most of the electron resides in a solvent cavity, as a non-cavity object, with many water molecules packed within the electron's wavefunction, or something in between. To address this question, we performed a series of mixed quantum/classical simulations with the goal of connecting the structure of the simulated hydrated electron with ultrafast spectroscopy experiments. We find that traditional cavity models are unable to predict the temperature dependence of the hydrated electron's excited-state lifetime, whereas a non-cavity model provides excellent agreement with temperature-dependent pump-probe experiments. Cavity models also fail to reproduce features of both static and ultrafast time-resolved photoelectron spectroscopy (TRPES) experiments, including observations such as the facts that hydrated electrons are not found near the air/water interface, that solvation dynamics lower the excited-state energy prior to internal conversion, and that solvation of the ground-state is significantly slower than the excited-state lifetime, features that are all well captured with a non-cavity model. |
Monday, March 5, 2018 12:27PM - 1:03PM |
B25.00003: Nonadiabatic Dynamics and Nested Funnels in Energy Transfer and Photosynthesis Invited Speaker: David Jonas In photosynthesis, antenna proteins harvest light and efficiently transfer the electronic excitation energy to reaction center proteins that initiate chemical transformations. The standard Forster framework for understanding electronic energy transfer is based on the adiabatic approximation of slow vibrations and fast electronic motions. Femtosecond two-dimensional spectra of many antenna proteins exhibit quantum beats with signatures indicative of nonadiabatic vibrational-electronic coupling. The longest lived beats originate from a nonadiabatic enhancement of Raman pathways for excitation of delocalized vibrations on the electronic ground state. These vibrations characterize the initial coupling between excited states and reveal nested funnels in which the adiabatic approximation fails spectacularly over entire vibrational coordinate spaces. The nonadiabatic dynamics in nested funnels is efficient at low vibrational energies and does not require the high vibrational velocities usually associated with breakdown of the adiabatic approximation. Nested funnel design principles will be discussed. |
Monday, March 5, 2018 1:03PM - 1:39PM |
B25.00004: Photoelectron processes in liquid water: new methods for probing elementary reactivity Invited Speaker: Stephen Bradforth In the condensed phase, intermolecular interactions often control chemical reactivity but are complex to quantify. Understanding how the electronic structure of an aqueous solute is intricately bound up with the arrangement of the host liquid provides insight into how reactions are influenced by the environment in which they take place. Our group has exploited liquid-jet photoelectron spectroscopy to learn about the orbital energies and shapes for water as well as for textbook aqueous inorganic ions in the bulk and at the air/water interface. With femtosecond time resolution, the non-equilibrium evolution of the valence orbitals involved in excited state and redox reactions can then be tracked. |
Monday, March 5, 2018 1:39PM - 2:15PM |
B25.00005: Abstract Withdrawn Invited Speaker: |
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