Sum-frequency vibrational spectroscopic studies of water/vapor interfaces

Phase-sensitive sum-frequency vibrational spectroscopy was used to obtain the OH and OD stretch spectra of the imaginary part of the nonlinear susceptibility, Im χ S ( 2 ) ( ω IR ) , for H2O/vapor, D2O/vapor, and 2:1 H2O–D2O/vapor interfaces that explicitly characterize their resonances. They all exhibit a positive and a negative resonance band in the bonded-OH region, clearly different from those deduced from fitting of the | χ S ( 2 ) ( ω IR ) | 2 spectra, and from MD simulations. The broad continuum is characteristic of inhomogeneously broadened resonances associated with interfacial water species of various hydrogen-bonding geometries and strengths. Spectral changes due to excess ions at the interfaces support the picture.