Relaxation dynamics of the hydrated electron studied with 5-fs pulses

Basically, the hydrated electron is an excess electron trapped in a potential well formed by surrounding water molecules, with an s-like ground state and three non-degenerate p-type excited states. The electron surrounded by the oriented water molecules is a chemical reactant with an unusually high electron donor capacity as its characteristic chemical feature. On the other hand, it seems to be one of the simplest physical systems to study solvation dynamics and to test mixed quantum classical theories experimentally. Yet, even after decades of intensive experimentation and calculations on the hydrated electron, understanding of its relaxation dynamics is far from being complete. One of the most important questions is the explanation of an ~1 ps relaxation rate of the photo-excited hydrated electron. This rate has been controversially attributed to the population lifetime of the p-state or cooling of the ground state after rapid relaxation from the p-state. We present the experimental study of the energy relaxation of the photo-excited hydrated electron. The results of frequency-resolved pump-probe with 5-fs pulses provide sufficient evidence in favor of the hot-ground-state model. The initial ultrafast energy relaxation of the photo-excited electron, controlled by the librations of the surrounding water molecules, takes place during the ~50 fs upon the excitation. We show that after the first 100 fs almost the entire population of the p-state is transferred to the hot ground state that subsequently cools down on a ps time scale