题 目：Probing elementary molecular events by ultrafast spectroscopy with coherent and noisy X ray pulses
报告人：Shaul Mukamel 院士
主持人：康司坦丁教授
时 间：2020年10月20日 下午2：00
地 点：腾讯会议 ID：919 256 643
报告人简介：
B. Sc. - Tel-Aviv University, 1969, cum laude; M. Sc. - Tel-Aviv University, 1971, summa cum laude;
Ph.D. - Tel-Aviv University, 1976, summa cum laude
July 2013-present: Distinguished Professor of Chemistry, University of California at Irvine; 2003-June 2013: Chancellor Professor of Chemistry, University of California at Irvine; 2002-2003:Professor of Physics and Astronomy, University of Rochester, Rochester, New York; 2000-2003:C. E. Kenneth Mees Professor of Chemistry, University of Rochester, Rochester, New York; 1985-2000: Professor, Department of Chemistry, University of Rochester, Rochester, New York; 1982-85: Associate Professor, Department of Chemistry, University of Rochester, Rochester, New York
报告内容简介：
Ultrafast nonlinear x-ray spectroscopy is made possible by newly developed free electron laser and high harmonic generation sources. The attosecond duration of X-ray pulses and the atomic selectivity of core X-ray excitations offer a uniquely high spatial and temporal resolution. We demonstrate how stimulated Raman detection of an X-ray probe may be used to monitor the phase and dynamics of the nonequilibrium valence electronic state wavepacket created by e.g. photoexcitation, photoionization and Auger processes. Conical intersections (CoIn) dominate the pathways and outcomes of virtually all photophysical and photochemical molecular processes. Short X-ray pulses can directly detect the passage through a CoIn with the adequate temporal and spectral sensitivity. The technique is based on a coherent Raman process that employs a composite femtosecond/attosecond X-ray pulse to directly detect the vibronic coherences (rather than populations) that are generated as the system passes through the CoIn. X ray sum frequency generation, and detecting molecular chirality. Coherences at conical intersections are probed by X-Ray stimulated Raman signals. Contrary to our common picture of a short femtosecond and precisely timed nonadiabatic passage, the coherence signature lingers for a much longer time.
Conical intersections (CoIns) are ubiquitous features on molecular potential energy surfaces (PESs) where the Born-Oppenheimer approximation breaks down. They determine the outcome of virtually all photochemistry and –physics, by opening non-radiative and ultrafast relaxation channels after optical excitation. Prominent examples include the primary initiation of vision or the ability of nucleobases in the genetic code to resist UV radiation. The existence of CoIns as fundamental features is widely accepted, yet their direct observation has so far eluded experimental efforts. Strong evidence for CoIns usually is based on the observation of ultrafast internal conversion rates via electronic state populations. Stimulated Raman probes of Uracil, with energy bandwidths able to resolve the potential energy splitting at the CoIn seam, map the electronic coherence at the CoIn in real time. The chaotic nature of x-ray free-electron-laser pulses is a bottleneck that has limited the joint temporal and spectral resolution of spectroscopic measurements. We show how to use the correlations of the stochastic x-ray field to overcome this difficulty by correlation signals averaged over independent pulse realizations. We further demonstrate that core excitations in molecules can be manipulated by placing them in X ray cavities, to form hybrid light matter excitations known as polaritons.
[1] Visualizing Conical Intersection Passages via Vibronic Coherence Maps Generated by Stimulated X-Ray Raman Signals”, Daniel Keefer, Thomas Schnappinger, Regina de Vivie-Riedle and Shaul Mukamel. PNAS (2020) http://arxiv.org/abs/2008.06191
[2] “High temporal and spectral resolution of stimulated x-ray Raman signals with stochastic free-electron-laser pulses”, Stefano M. Cavaletto, Daniel Keefer, and Shaul Mukamel. (In preparation)
[3] “Manipulating core-excitations in molecules by X-ray cavities”, Bing Gu, Artur Nenov, Francesco Segatta, Marco Garavelli, and Shaul Mukamel. (in preparation)