Hydrated-Electron Relaxation Dynamics Studied with 5-fs Pulses

Excess electrons in condensed-phase media play a crucial role in the dynamics of important chemical processes. Among those are solution photochemistry, nonradiative electronic transitions, and charge transfer reactions. Hydrated electrons, i.e. electrons solvated in water, are of special interest. They can be viewed as an exceptional instrument for extracting information about the solvation process in water that plays an outstanding role in nature. Another motivation for a detailed study of the hydrated electron stems from the unique possibility to confront the predictions of mixed classical-quantum mechanical molecular dynamics simulations. This presents a direct way to verify the basic a priori assumptions that radically influence the outcome of computer modeling. We present an experimental study of the energy relaxation of the photoexcited hydrated electron. The results of frequency-resolved pump-probe with 5-fs pulses provide sufficient evidence in support of an extremely fast initial energy relaxation. Our data show that this process is controlled by librations of the surrounding water molecules and has a decay time of ~50 fs. We further demonstrate that the subsequent cooling of the hot-ground state proceeds on a ps time scale and exhibits no isotope effect.