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 chiS(2) spectra, where chiS(2) is the surface nonlinear susceptibility, permit direct characterization of resonances of the interfaces. The Im chiS(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 |chiS(2)|2 spectra and those calculated by MD simulation, but consistent with that obtained for the H2O/vapor interface.