Understanding the population, coordination, and orientation of water species contributing to the nonlinear optical spectroscopy of the vapor-water interface through molecular dynamics simulations.

Molecular dynamics simulations are used to deconvolve the vibrational spectral features of the vapor-water interface based on molecular environment. A simple geometric description of hydrogen bonding is deployed to identify the OH stretch modes that comprise the vibrational sum-frequency spectrum of the vapor-water interface with direct comparison to our experimental results. The population densities of different species of water molecules are presented as functions of interfacial depth and orientation. It is found that surface water molecules that possess one proton donor bond and one proton acceptor bond make the dominant contribution to both the SSP- and SPS-polarized spectral responses and are located within an angstrom of the Gibbs dividing surface.