Vibrational Spectroscopic Response of Intermolecular Orientational Correlation at the Water Surface

The second-order nonlinear susceptibility, which is the source of the sum frequency generation (SFG) spectrum, for the vapor−water interface is calculated by molecular dynamics simulation based on the time correlation formula. The calculated susceptibility well reproduces that of the recent phase-sensitive SFG experiment (Ji et al. Phys. Rev. Lett. 2008, 100, 096102), including the positive imaginary part in the 3000−3200 cm −1 region, which is a controversial problem in the assignment. It is found that this positive part of the imaginary susceptibility is induced by the intermolecular orientational correlation at the interface through the anisotropic local field.