The analysis of interference effects in the sum frequency spectra of water interfaces

Vibrational sum frequency (VSF) spectroscopy has been increasingly used in recent years to measure the vibrational spectroscopy of molecules at surfaces. Some of the more important systems examined in such studies involve the surface of liquid water. Unfortunately, obtaining spectral fits to vibrational spectra acquired in these studies can be challenging. The difficulty arises from the wide range of contributing vibrational modes, the breadth of the spectral peaks for hydrogen-bonded water molecules, and the complex interference effects that can arise between adjacent vibrational modes because of the coherent nature of the sum frequency process. In this paper, we describe a detailed method for obtaining spectral fits to such VSF data, that takes into account a range of water species present at a water surface and die possible interference between these contributing modes. The relationship between the spectral information derived and the molecular orientation is also given. This analysis is applied to two cases, VSF spectra of water measured at the CCl4/H2O interface and at the air/H2O interface.