Considerations of optical anisotropy in the simulation of reflection absorption infrared spectra

Simulations of multilayer reflection-absorption infrared spectra (RAIRS) are commonly based upon the application of Greenler equations which treat each layer as being optically isotropic. Extensions of the theory to include anisotropic adsorbate layers are also available but discussion of anisotropy in subsequent substrate layers have largely been ignored. In this paper, based on known principles, we develop a general theory for the simulation of RAIR spectra taking into account optical anisotropy in any or all layers of a many-layered system. The effect of considering optical anisotropy is illustrated by a four-layer system (typical when using the so called buried metal layer BML method) where adsorbate-induced substrate-derived bands may be observed. We show that in such cases, the effect of optical anisotropy may significantly alter the appearance of the spectrum.