Vibrational sum-frequency spectroscopy of the liquid/vapor interface for dilute HOD in D2O

An electronic structure/molecular dynamics approach, originally developed to describe the vibrational spectroscopy of the OH stretch of dilute HOD in liquid D2O, is applied to the vibrational sum-frequency spectroscopy of the liquid/vapor interface of this system. In both cases the OH stretch is effectively decoupled from the OD stretches, allowing it to act as a local probe of structure and dynamics. A mixed quantum/classical expression for the vibrational sum-frequency response that includes the effect of motional narrowing is used to calculate the resonant susceptibility. Despite being developed for the bulk liquid, our method works well for the surface in that the real and imaginary parts of the resonant susceptibility are in good agreement with experiment. We explore the nature of hydrogen bonding at the interface as well as its impact on the sum-frequency spectrum. It is found that the spectrum is dominated by single-donor molecules with a total of two or three hydrogen bonds.