Changes in the infra-red spectra of molecules due to physical adsorption

Abstract

Infra-red spectra of the molecules methane, ethylene, acetylene and hydrogen, physically adsorbed on porous silica glass have been studied at measured surface coverages, θ . In all cases absorption bands were readily obtained for θ values considerably less than unity; for methane it was possible to obtain spectra down to θ = 0.01. Shifts occurred on adsorption for the absorption bands that were observed in the gas phase and in the adsorbed state. In several cases additional bands occurred in the spectra of the adsorbed molecules which correspond to known fundamental vibration frequencies that are forbidden in the gas phase because of molecular symmetry. Their appearance results directly from the distortion of the symmetry of the molecules by surface forces. High-resolution spectra of adsorbed methane and hydrogen obtained with a diffraction grating spectrometer enabled the shapes of the bands to be accurately determined. Although no fine-structure lines due to molecular rotation could be resolved, the overall shape of the broad and asymmetrical v 3 band of methane could be explained by using as a theoretical model free rotation of the molecule about a single axis normal to the surface. The band shape is not consistent with a three-dimensional free rotation about axes parallel as well as perpendicular to the surface. It could, however, alternatively be fitted by a Lorentz curve of half-width 28 cm -1 , corresponding to a broad vibration band without rotational contribution, plus an extra overtone absorption band near 3050 cm -1 . The shapes of the other bands of adsorbed methane and hydrogen are insensitive to the assumptions of free or highly restricted rotation on the surface. A comparison of the intensity of the absorption band due to adsorbed hydrogen with that of gaseous hydrogen in a strong electrical field (Crawford & Dagg 1953) has enabled an estimate to be made of the electrostatic field at the surface of the glass.