Momentum-Dependent Nonlinear Optical Spectroscopy of Charged Aqueous Interfaces

Interfacial hydrogen (H)–bonding network dominates energy transfer and reaction pathway at charged aqueous interfaces. While sum-frequency (SF) spectroscopy is versatile to investigate interfacial systems for its high interface sensitivity, correct SF spectral interpretation on charged water interfaces remains challenging for years. The difficulty arises from the third-order nonlinear optical effect owing to the dc surface field formed by interfacial charges and ions in the electrical diffuse layer. Here we report a momentum-dependent sum-frequency vibrational spectroscopic scheme for extracting the interfacial SF spectra with the third-order effect excluded. Applying the method to a model surfactant-water interface reveals a hidden weakly donor H-bonded water species, suggesting an asymmetric hydration-shell structure of fully solvated surfactant headgroups. In another application to a zwitterionic phosphatidylcholine lipid monolayer–water interface, we find a highly polarized H-bonded water structure associating to the phosphatidylcholine headgroup, and show that the dc-field-induced third-order effect is negligible by experiments. Our all-optic method offers an in situ microscopic probe of electrochemical and biological interfaces.