Phase-sensitive sum frequency vibrational spectroscopic study of air/water interfaces: H₂O, D₂O, and diluted isotopic mixtures

Using phase-sensitive sum-frequency vibrational spectroscopy with a carefully chosen phase reference, we revisited the vibrational spectra of vapor/water interfaces of neat H₂O and D₂O as well as HDO in diluted isotopic mixtures. Using z-cut quartz as the phase reference, with proper frequency scaling, the gross features of the OH and OD stretching spectra from H₂O and D₂O and from HDO in two isotopic mixtures look similar and agree with those reported earlier, but differences are also apparent. In particular, a weak positive band at low frequencies, which has been asserted by molecular dynamic simulations but not detectable in the experimental OH spectrum with pure H₂O, is now visible in the OD spectrum. The differences must arise from the change of intermolecular interaction of water molecules with their surrounding molecules upon exchange of the isotopes.