Electron Affinity of Liquid Water*

Understanding redox reactions in aqueous environments requires a precise knowledge of the ionization potential and electron affinity of liquid water. The former has been measured, but the latter remains unknown. We predict the electron affinity (EA) of liquid water and of its surface from first principles, coupling path-integral molecular dynamics with ab initio potentials and many-body perturbation theory calculations. Our results for the surface agree with recent pump-probe spectroscopy measurements, while those for the bulk differ from several estimates adopted in the literature. We revisit these estimates and present an updated energy diagram for an electron in water. We show that the ionization potential of the bulk and surface are almost identical; instead their EAs differ substantially, with the conduction band edge of the surface much deeper in energy than that of the bulk. We also discuss the significant impact of nuclear quantum effects on the fundamental gap and band edges of the liquid.

A. P. Gaiduk, T. A. Pham, M. Govoni, F. Paesani, and G. Galli, Nat. Commun. (submitted)