A theoretical study of inverse features in far-infrared RAIRS spectra from non-metallic substrates

A theoretical investigation of far-infrared RAIRS spectra of adsorbates on a non-metallic substrate has been carried out using wavenumber-dependent Greenler type calculations. Using a three-layer model (vacuum, adsorbate, oxide), these calculations show that, in addition to the well-known effect of adsorbate vibrational bands changing directions as the angle of incidence passes through the pseudo-Brewster angle, other spectral features related to the wavenumber-dependent refractive index of the substrate may appear. These features are unrelated to the vibrational properties of the adsorbate but relate to strong dips in reflectivity of the substrate. The experimental observation of such bands is difficult to predict, however, since poor reflectivity will result in loss of signal. The results are compared with those from a four-layer model (vacuum, adsorbate, thin oxide, metal) and the experimental implications are discussed.