Isotopic dilution studies of the vapor/water interface as investigated by vibrational sum-frequency spectroscopy

Vibrational sum-frequency spectroscopy (VSFS) studies of dilute HOD/H2O/D2O mixtures have been used to investigate the hydrogen-bonding interactions at the vapor/water interface. The decoupled nature of the HOD vibrations produces a less complex OH stretching band than the highly coupled OH stretches of H2O, creating a clearer picture of the molecular environments present in the interfacial region. From the spectra of interfacial HOD, it is determined that the frequency of the uncoupled donor OH stretching mode is similar to what is observed in bulk HOD studies, leading to the conclusion that the hydrogen bonding environment at the surface is similar to that found in liquid H2O. This conclusion is supported by the tetrahedrally coordinated region of the vapor/H2O VSF spectrum, where the predominant resonant intensity is centered around 3300 cm(-1), consistent with bulk Raman and IR spectra of liquid water.