Enhancement of the Surface Modes in Photoacoustic FT-IR Spectroscopy Using a Highly Polarizable Inert Gas

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Abstract
Fourier transform infrared photoacoustic spectroscopy has been used for the analysis of surface functionality and adsorbed species. Using a highly polarizable gas in the photoacoustic cell and comparing the spectra with a nonpolarizable coupling gas, one can obtain useful information regarding the species present and their orientation with respect to the surface. A highly polarizable inert gas, xenon, enhances those surface modes which are preferentially oriented parallel to the surface and suppresses the intensity of the perpendicular modes. The two types of modes for adsorbed molecules on the oxide surfaces such as silica hare been demonstrated. The intensities of the carbon monoxide mode (parallel to the surface) are greatly enhanced and those of adsorbed water (perpendicular) are suppressed. Surface hydroxyls show less intensity in xenon than helium, reflecting preferentially perpendicular orientation with respect to the surface. This method requires only a routine photoacoustic setup and the use of polarizable and nonpolarizable gases.