Solvent mode participation in the non-radiative relaxation of the hydrated electron

Abstract

Non-adiabatic molecular dynamics simulations are used to analyze the role of different solvent degrees of freedom in the non-radiative relaxation of the first excited state of the hydrated electron. The relaxation occurs through a multi-mode coupling between the adiabatic electronic states. The process cannot be described by a single mode promotion model frequently used in the ``large molecule'' limit of gas phase theories. Solvent librations and vibrations, and the H2O asymmetric stretch in particular, are found to be the most effective promotors of the electronic transition. Dissipation of the released energy to the solvent proceeds on two time scales: a fast 10-20 fs heating of the first solvation shell, where most of the energy is accepted by the librational degrees of freedom, and a several hundred femtosecond global reconstruction of t he solvent as the first shell transfers its excess energy to the rest of the molecules. The implications of our use of a semiclassical approximation as the criterion for good promoting and accepting modes are discussed.