Probing the Electronic Structure of Bulk Water at the Molecular Length Scale 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 bands of neat water using X-ray radiation (250-750 eV) show a limited (∼20%) decrease in the β anisotropy parameter compared to the gas phase, indicating that the electronic structure of the individual water molecules can be probed. We show that, in the high-energy regime, photoionization of bulk can be described using 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 versus high energy-are limiting cases where the de Broglie wavelength of the photoelectron is larger or smaller than the intermolecular distance between water molecules, respectively. The comparison of measured and computed anisotropies reveals that the reduction in β at high kinetic energies is mostly due to scattering rather than rehybridization due to solvation.