Simulating the Photochemical Birth of the Hydrated Electron in Liquid Water.

The photochemistry of UV-irradiated liquid water underlies many physical, chemical, and biological processes, with the formation of the hydrated electron as a central event. Despite extensive experimental and theoretical efforts, its microscopic origin remains incompletely understood. Using excited state molecular dynamics simulations of photoexcited liquid water, we resolve the sequence of chemical events leading to hydrated electron formation on the excited state. The excitation localizes on specific hydrogen-bond network defects, followed by two competing pathways. The first produces a hydrogen atom and undergoes ultrafast non-radiative decay to the ground state within 100 femtoseconds. The other proceeds via proton-coupled electron transfer, generating hydronium ions, hydroxyl radicals, and an excited state hydrated electron. This mechanism is driven by ultrafast coupled rotational and translational motions of water molecules, forming water-mediated ion-radical pairs that persist on picosecond timescales and influence visible emission. These results provide a unified framework for interpreting time-resolved spectroscopic observations and guide future experimental and theoretical investigations.