A theoretical investigation of the far-infrared RAIRS experiment applied to a buried metal layer substrate

In this paper we report a theoretical investigation of the reflection absorption infrared spectroscopy (RAIRS) experiment applied to non-metallic substrates incorporating a buried metal layer (BML). In particular we concentrate on the low wavenumber region (280–480 cm−1) typically studied using synchrotron based RAIRS experiments. To illustrate this study, an example system consisting of a hypothetical isotropic layer adsorbed on thin-films of tin(IV) oxide on tungsten has been used. Using a four-layer model, systematic wavenumber-dependent Greenler type calculations of ΔR/R (often taken as a measure of the sensitivity of the experiment) have been performed. The calculated spectra show the absorption band associated with the adsorbate but also reproduce an “inverse absorption” feature, which has been observed experimentally. The results show that the factors which influence the intensity of these adsorbate-induced inverse absorption bands are different from those that influence adsorbate bands and may therefore be used to discriminate between these two types of feature. A partial first order approximation of the full Greenler-type formula has then been used to rationalise the calculated dependences of the intensities of both absorption and inverse absorption bands and system parameters like layer thicknesses, incidence angles and optical characteristics of the adsorbate.