Real-time structural dynamics of the ultrafast solvation process around photo-excited aqueous halides†
*a
Jaysree
Pan,
a
Bianca L.
Hansen,
a
Morten L.
Haubro,
a
Amke
Nimmrich,
‡ab
Matteo
Levantino,
c
Tetsuo
Katayama,
de
Friedrich
Temps,
h
Asmus O.
Dohn,
ai
Klaus B.
Møller,
a
Martin M.
Nielsen
a
and
Kristoffer
Haldrup
a
Abstract
This work investigates and describes the structural dynamics taking place following charge-transfer-to-solvent photo-abstraction of electrons from I− and Br− ions in aqueous solution following single- and 2-photon excitation at 202 nm and 400 nm, respectively. A Time-Resolved X-ray Solution Scattering (TR-XSS) approach with direct sensitivity to the structure of the surrounding solvent as the water molecules adopt a new equilibrium configuration following the electron-abstraction process is utilized to investigate the structural dynamics of solvent shell expansion and restructuring in real-time. The structural sensitivity of the scattering data enables a quantitative evaluation of competing models for the interaction between the nascent neutral species and surrounding water molecules. Taking the I0–O distance as the reaction coordinate, we find that the structural reorganization is delayed by 0.1 ps with respect to the photoexcitation and completes on a time scale of 0.5–1 ps. On longer time scales we determine from the evolution of the TR-XSS difference signal that I0: e− recombination takes place on two distinct time scales of ∼20 ps and 100 s of picoseconds. These dynamics are well captured by a simple model of diffusive evolution of the initial photo-abstracted electron population where the charge-transfer-to-solvent process gives rise to a broad distribution of electron ejection distances, a significant fraction of which are in the close vicinity of the nascent halogen atoms and recombine on short time scales.
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