Many-Body Screening Drives Nanoscale Core-Level Variation at the Water Interface.

The depth at which the interfacial behavior of liquid water transitions into bulk behavior is critical toward understanding atmospherically relevant processes. Using intrinsic chemical shifts of O1s core levels, we demonstrate that the bulk electronic behavior emerges within 6 Å of the vacuum-liquid interface. This rapid onset is in remarkable agreement with observations from nonlinear vibrational spectroscopy yet has a different physical origin of electronic dielectric screening. Within the interfacial region, we observe an intrinsic core-level bending of 1.0 eV. Using many-body methods, we show that the level bending is driven by differences in optical dielectric screening, which is a final state effect, in the interfacial region compared to the bulk rather than by differences in the hydrogen-bonding network. However, the local hydrogen-bonding motifs majorly contribute to the distribution of shifts in liquid water. On the vacuum side, the O1s core-level energies attain the gas-phase values at a distance of 10 Å from the interface. We discuss the implications of our findings toward obtaining depth resolution of solutes at liquid interfaces.