VIBRATIONAL-SPECTRUM OF WATER AT THE LIQUID/VAPOR INTERFACE

The infrared spectrum of water molecules at the liquid/vapor interface of pure water and at the liquid/vapor interface of a water-methanol mixture is calculated using molecular dynamics. A comparison with the IR spectrum calculated for bulk water shows a significant population of free OH bonds, in qualitative agreement with a recent experiment by Du et al. [Phys. Rev. Lett. 70, 2313 (1993)]. It is also demonstrated that the free OH IR signal is quenched by a moderate increase in the concentration of methanol in bulk water, which has been recently observed by Du et al. [Science 264, 826 (1994)].