Isotopic Dilution Study of the Water/Vapor Interface by Phase-Sensitive Sum-Frequency Vibrational Spectroscopy

Phase-sensitive sum-frequency vibrational spectroscopy (PS-SFVS) has been used to probe the isotopically diluted water/vapor interfaces in the spectral. regions of OD (2200-2800 cm(-1)) and OH (3000-3800 cm(-1)) stretches. The experimentally measured Im x(s)((2)) spectra, where x(s)((2)) is the surface nonlinear susceptibility, permit direct characterization of resonances of the interfaces. The Im x(s)((2)) spectrum of the HDO/vapor interface that is intrinsically simpler to analyze can be deduced from the result of isotopic dilution. It exhibits in the bonded-OH region a broad band comprising two parts with opposite signs, in contrast to those deduced earlier from fitting of the vertical bar x(s)((2))vertical bar(2) spectra and those calculated by MD simulation, but consistent with that obtained for the H2O/vapor interface.