Probing ultrafast electronic and hydrogen dynamics with ultrafast electron diffraction and transient X-ray absorption Nanna Holmgaard List 1 , Alice E. Green 1 , Elio Champenois 1 , Pratip Chakraborty 2 , Matthew Ware 1 , Taran Driver 1,3 , Andrey Boguslavskiy 4, Phil Bucksbaum 1, Xinxin Cheng 3, Giacomo Coslovich 3, Ruaridh Forbes 1,3 , James M. Glownia 3 , Markus Guehr 5 , Andrei Kamalov 3 , Fabiano Lever 5 , Siqi Li 3, Xiang Li 3 , Ming-Fu Lin 3 , Mathew Britton 1, Martin Centurion 6, Ian Gabalski 1, Kareem Hegazy 1, Matthias C. Hoffmann 3 , Andrew J. Howard 1,3 , Fuhao Ji 3 , Pedro Nunes 6 , Xiaozhe Shen 3 , Xijie Wang 3 , Todd J. Martinez 1 , Dennis Mayer 5 , Jordan O’Neal 1 , Nolan Peard 1 , Anja Roeder 4 , Albert Stolow 4 , Peter Walter 3 , Anna L. Wang 1 , Jie Yang 3 , James Cryan 1,3 , Nanna H. List 2 , Thomas J.A. Wolf 1,3 1 Stanford PULSE Institute, SLAC National Accelerator Laboratory, Menlo Park, CA, USA, 2 Department of Chemistry, KTH Royal Institute of Technology, Stockholm, Sweden, 3 LCLS, SLAC National Accelerator Laboratory, Menlo Park, CA, USA, 4 Department of Physics, University of Ottawa, Ottawa, Ontario, Canada, 5 Institut für Physik und Astronomie, Universität Potsdam, Potsdam, Germany, 6 Department of Physics and Astronomy, University of Nebraska Lincoln, Lincoln, USA Photochemical hydrogen and proton transfer reactions are among the fastest chemical reactions and are ubiquitous in chemistry and biology. However, directly following the ultrafast hydrogen and proton dynamics with time-resolved experimental methods is complicated by the fast time scales down to the few-fs regime. Here we present the results of two recent theory–experiment collaborations aimed at unveiling photoinduced hydrogen motion using X-ray or relativistic electron probes. First, and motivated by the results of our initial in silico pump–probe experiment on malonaldehyde, 2 we employ transient X-ray absorption spectroscopy at the oxygen K-edge to probe the ultrafast behavior of the methylated derivative acetylacetone upon excitation to S 2 (ππ ∗ ). A variety of techniques have been used to study the photodynamics of acetylacetone (involving intersystem crossing and fragmentation) 3 – yet the mechanism underlying its earliest response has so far remained elusive. Our nonadiabatic dynamics simulations suggest an ultrafast intermolecular hydrogen transfer motion coupled with internal conversion to the S 1 (nπ ∗ ) state, which leads to distinct changes in the oxygen K-pre-edge features. Indeed, our experimental difference signal reveals ultrafast signatures occurring on sub-100-fs time scales, indicative of nonadiabatic dynamics involving excited-state hydrogen transfer. An interesting aspect under further investigation is the role of the seemingly innocent methyl groups in the ultrafast response. 4 Second, we probe the sensitivity of megaelectron-volt ultrafast electron diffraction to follow concerted electronic and hydrogen dynamics in isolated molecules by studying the photodissociation dynamics of gas-phase ammonia. Combined with our time-resolved ab initio scattering simulations, we demonstrate the simultaneous imaging of both the electronic and nuclear components in distinct regions of momentum space. 5 While the temporal resolution of our experiment is insufficient to resolve the dissociation in time, our results present an important step toward the observation of proton dynamics in real space and time. References 1. N. H. List, A. et al., Chem. Sci. 2020 , 11, 4180.
2. Antonov et al., JPCA 2019 , 123, 5472; 3. Squibb et al., Nat. Comm. 2018 , 9, 63; 4. Bhattacherjee et al., JACS 2017 , 139, 46, 16576. 5. A. Green et al., in preparation.E. G. Champenois et al., in preparation.
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