David J. Hoffman, Shane W. Devlin, Douglas Garratt, Sasawat Jamnuch, Jacob A. Spies, Bailey R. Nebgen, Daniel Schacher, Alexandria Do, Franky Bernal, Erika J. Riffe, Kristjan Kunnus, Christina Y. Hampton, Joseph Duris, David Cesar, Nicholas Sudar, Georgi L. Dakovski, Walter S. Drisdell, Keith V. Lawler, Agostino Marinelli, Michael Zuerch, Richard J. Saykally, Craig P. Schwartz, Tod A. Pascal, J. D. Koralek
The microscopic structure of water's surface is crucial to many natural and industrial processes, but studying its hydrogen bond (H-bond) network directly remains challenging due to the required interfacial sensitivity of experimental techniques. By leveraging advances in flat liquid sheet microjets and terawatt-scale attosecond soft X-ray pulses from the LCLS X-ray free electron laser, we employed soft X-ray second harmonic generation (SXSHG) spectroscopy to examine the liquid water/vapor interface. SXSHG combines the elemental selectivity of X-ray spectroscopies with the surface selectivity of SHG and gives access to the electronic structure of interfacial species. Here, we show the SXSHG spectrum differs from bulk water's X-ray absorption, with its peak shifted several eV, indicating a vastly different electronic environment at the interface as compared to the bulk. First-principles electronic structure calculations show the signal is highly sensitive to H-bond interactions, such as water molecules accepting a single H-bond, which are surface abundant.