Effects of dipolar coupling on the intensity of infrared absorption bands from supported metal catalysts

Abstract

Although the influence of dipole coupling on spectra from adsorbed species has been much investigated in recent years, most of this work has concentrated on the frequency shifts induced by coupling, while the effects on intensity have been largely ignored. However, coupling can produce such profound changes in band intensities that serious mis-interpretation of spectra may arise if its influence is ignored: this is especially true for spectra obtained from relatively ill-defined substrates such as supported catalysts. Both absolute and relative band intensities are affected by coupling. Depolarization effects reduce molecular absorption intensities at high adsorbate densities, often very significantly; a reduction by a factor of 4 occurs with carbon monoxide molecules at a coverage of 10 15 cm −2. However, this influence of coupling is likely to be less important than the concomitant changes in the relative intensities of bands. These arise when two or more species with similar vibrational frequencies occur in physical proximity, and they then enhance the intensity of bands at high wavenumber, sometimes to such a degree that a species present to the extent of less than 5 % of the total may come to dominate the spectrum. Experiments have shown that slight variations in structure caused by surlhce defects can give rise to dramatic changes attributable to this effect.