EXAFS probing of nonequilibrium solvation dynamics of nascent halogen radicals

Extended X-ray absorption fine structure (EXAFS) spectroscopy can be used to observe the local environment around absorber atoms. Provided high enough time resolution can be achieved, this would open the possibility of tracking nonequilibrium solvation dynamics in photoreactions in real time. Here we simulate an electron detachment process from aqueous Cl−, Br−, and I− using quantum mechanics/molecular mechanics molecular dynamics simulations and analyze the following solvation dynamics. We show how femtosecond-resolved EXAFS spectra, calculated using an effective dissipative theory, capture the solvation dynamics around nascent Cl0, Br0, and I0. The atomistic simulations reveal that, after instantaneous electron detachment, the rotation of water molecules in the solvation shell as well as the following formation of a halogen-atom–water complex happens more slowly for smaller halogen species. Here the initially tighter and more ordered solvation shell around the smaller halogen ions hinders the nonequilibrium rearrangement of the water molecules, in response to the change in charge state, compared to the larger halogens. Even though directly recovering atom distributions during the solvation process by Fourier inversion of EXAFS spectra was not possible, the time-dependent EXAFS spectrum is sensitive to nonequilibrium dynamics, taking into consideration that the phase shifts experienced by the EXAFS photoelectron before and after photodetachment from halide anions are different. In addition we demonstrate that it is possible to discern the nonequilibrium solvation dynamics for different nascent halogen atoms using time-dependent EXAFS.