Vibrational sum-frequency spectroscopy of the water liquid/vapor interface.

We present theoretical calculations of the vibrational sum-frequency susceptibility for the water liquid/vapor interface. Our approach builds on previous calculations by us and others, using the time-averaging approximation within the mixed quantum/classical formulation for coupled vibrational chromophores, and electric-field maps for transition frequencies, dipoles, polarizabilities, and intramolecular vibrational couplings. We compare our results for the imaginary part of the susceptibility to those from recent experiments, and comment about the effects of intermolecular vibrational coupling and the assignment of features in the spectrum.