Real-time structural dynamics of the ultrafast solvation process around photo-excited aqueous halides

Time-resolved X-ray solution scattering with a 1-photon and 2-photon pump qualitatively confirms the ∼0.5 Å increase in the nearest-neighbour halide–oxygen distances of iodide and bromide solvated in water. 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 I 0 –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 I 0 : 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.