Probing the Electronic Structure of Bulk Water at the Molecular Lengthscale with Angle-Resolved Photoelectron Spectroscopy

We report a combined experimental and theoretical study of bulk water photoionization. Angular distributions of photoelectrons produced by ionizing the valence band of neat water using X-ray radiation (250-750 eV) show a limited (<30 %) decrease in the beta anisotropy parameter compared to the gas phase, indicating that the electronic structure of the individual water molecules can be probed. By theoretical modeling using high-level electronic structure methods, we show that in a high-energy regime photoionization of bulk can be described as an incoherent superposition of individual molecules, in contrast to a low-energy regime where photoionization probes delocalized entangled states of molecular aggregates. The two regimes-low energy versus high energy-are defined as limiting cases where the de Broglie wavelength of the photoelectron is either larger or smaller than the intermolecular distance between water molecules, respectively. The comparison of the measured and computed anisotropies reveals that at high kinetic energies the observed reduction in beta is mostly due to scattering rather than rehybridization due to solvation.