The Photochemical Birth of the Hydrated Electron in Liquid Water

TL;DR

This study uses molecular dynamics simulations to elucidate the sequence of events leading to hydrated electron formation in water upon UV irradiation, revealing defect localization and reaction pathways.

Contribution

It provides a detailed mechanistic understanding of hydrated electron generation in liquid water, combining simulation insights with experimental spectroscopy interpretations.

Findings

Excitation occurs on topological defects in water's hydrogen-bond network.

Two main pathways: non-radiative decay or proton-coupled electron transfer.

Formation of water-mediated ion-radical pairs influences spectroscopic signals.

Abstract

The photophysics and photochemistry associated with irradiating UV light in liquid water is central to numerous physical, chemical and biological processes. One of the key events involved in this process is the generation of the hydrated electron. Despite long study from both experimental and theoretical fronts, a unified understanding of the underlying mechanisms associated with the generation of the solvated electron have remained elusive. Here, using excited-state molecular dynamics simulations of condensed phase photoexcited liquid water, we unravel the key sequence of chemical events leading to the creation of the hydrated electron on the excited state. The process begins through the excitation localized mostly on specific topological defects in the hydrogen-bond network of water which is subsequently followed by two main reaction pathways. The first, leads to the creation of a hydrogen atom culminating in non-radiative decay back to the ground-state within 100 femtoseconds. The second involves a proton coupled electron transfer, giving rise to the formation of the hydronium ion, hydroxyl radical and the hydrated excess electron on the excited-state. This process is facilitated by ultrafast coupled rotational and translational motions of water molecules leading to the formation of water mediated ion-radical pairs in the network. These species can survive on the picosecond timescale and ultimately modulate the emission of visible photons. All in all, our findings provide fresh perspectives into the interpretation of several independent time-dependent spectroscopies measured over the last decades, paving the way for new directions on both theoretical and experimental fronts.