Surface structure of water from soft X-ray second harmonic generation
- 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 W. Zuerch
- Richard J. Saykally
- Craig P. Schwartz
- Tod A. Pascal
- Jake D. Koralek
2025-11-26
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.