We present a sub-picosecond resolved investigation of the structural solvent reorganization and geminate recombination dynamics following 400 nm two-photon excitation and photodetachment of a valence p electron from the aqueous atomic solute, I−(aq). The measurements utilized time-resolved X-ray Absorption Near Edge Structure (TR-XANES) spectroscopy and X-ray Solution Scattering (TR-XSS) at the Linac Coherent Light Source x-ray free electron laser in a laser pump/x-ray probe experiment. The XANES measurements around the L1-edge of the generated nascent iodine atoms (I0) yield an average electron ejection distance from the iodine parent of 7.4 ± 1.5 Å with an excitation yield of about 1/3 of the 0.1M NaI aqueous solution. The kinetic traces of the XANES measurement are in agreement with a purely diffusion-driven geminate iodine–electron recombination model without the need for a long-lived (I0:e−) contact pair. Nonequilibrium classical molecular dynamics simulations indicate a delayed response of the caging H2O solvent shell and this is supported by the structural analysis of the XSS data: We identify a two-step process exhibiting a 0.1 ps delayed solvent shell reorganization time within the tight H-bond network and a 0.3 ps time constant for the mean iodine–oxygen distance changes. The results indicate that most of the reorganization can be explained classically by a transition from a hydrophilic cavity with a well-ordered first solvation shell (hydrogens pointing toward I−) to an expanded cavity around I0 with a more random orientation of the H2O molecules in a broadened first solvation shell.
Tracking structural solvent reorganization and recombination dynamics following e− photoabstraction from aqueous I− with femtosecond x-ray spectroscopy and scattering
Current address: Department of Physics, Technical University of Denmark, Fysikvej 307, DK-2800 Kongens Lyngby, Denmark and Science Institute and Faculty of Physical Sciences, University of Iceland, 107 Reykjavík, Iceland.
Current address: Department of Physics, University of Wisconsin-Madison, 1150 University Avenue, Madison, WI 53706, USA.
Peter Vester, Katharina Kubicek, Roberto Alonso-Mori, Tadesse Assefa, Elisa Biasin, Morten Christensen, Asmus O. Dohn, Tim B. van Driel, Andreas Galler, Wojciech Gawelda, Tobias C. B. Harlang, Niels E. Henriksen, Kasper S. Kjær, Thomas S. Kuhlman, Zoltán Németh, Zhangatay Nurekeyev, Mátyás Pápai, Jochen Rittman, György Vankó, Hasan Yavas, Diana B. Zederkof, Uwe Bergmann, Martin M. Nielsen, Klaus B. Møller, Kristoffer Haldrup, Christian Bressler; Tracking structural solvent reorganization and recombination dynamics following e− photoabstraction from aqueous I− with femtosecond x-ray spectroscopy and scattering. J. Chem. Phys. 14 December 2022; 157 (22): 224201. https://doi.org/10.1063/5.0107224
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