Nonlinear optical studies of the pure liquid/vapor interface: Vibrational spectra and polar ordering

Using infrared-visible sum-frequency generation we have obtained the vibrational spectra of CH stretches of methanol molecules at the interface between methanol vapor and liquid. This is the first vibrational spectrum ever observed from a neat liquid surface. The measured polarization dependence and phase of the nonlinear susceptibility allow us to conclude that the surface methanol molecules are polar oriented with the ${\mathrm{CH}}_3$ group pointing away from the liquid with a very broad orientational distribution.