Time resolved solvent rearrangement dynamics

Abstract

Ultrafast pump–probe studies of I₂⁻ recombination in size-selected I₂⁻(CO₂)_n cluster ions demonstrate long time coherent motions in size-selected clusters and the resulting non-statistical energy flow in the cluster. For I₂⁻ photodissociated to produce either I⁻ + I or I⁻ + I*, we identify a solvent-driven energy transfer process without a condensed phase counterpart. The mechanism involved is a Marcus-like solvent-driven curve crossing, with the driving force arising from asymmetric solvation, not just from solvent orientation. By substituting another halogen for one I atom, we “break” the I₂⁻ symmetry, and thus obtain direct information on the electron transfer process. New experiments on IBr⁻(CO₂)_n photodissociation products confirm the behavior suspected in the I₂⁻ studies. Time-resolved experiments on IBr⁻ and theoretical modeling of the dynamics provide quantitative information on the multiple curve crossings encountered in the recombination process. In related investigations, femtosecond negative ion-neutral-positive ion charge reversal apparatus is employed to investigate transient neutral species evolving along a reaction coordinate. We report studies of the rearrangement dynamics of Cu(OH₂) produced by photodetachment of the corresponding anion. Following a controlled delay period, a second ultrafast tunable laser pulse initiates resonant multiphoton photoionization of the time-evolving Cu⋯OH₂ complex. The time-resolved Cu⁺ and Cu⁺(OH₂) signals provide information both on the prompt dissociation of the complex and on energy redistribution between internal rotational and radial modes of the evolving complex. Calculations of the time evolution of the anion geometric configuration on the neutral potential energy surface yield deeper insight into the nature of the rearrangement process and the energy flow within the complex.