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 chi((2))(s) (omega(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 vertical bar chi((2))(s) (omega(IR))vertical bar(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. (C) 2009 Elsevier B.V. All rights reserved.