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