Theoretical aspects of differential reflectance and electroreflectance spectroscopy in the UV-Vis region as applied to the study of molecular layers adsorbed on metal surfaces

Differential reflectance UV-vis spectra (ΔR/R vs wavelength, λ) of molecular layers adsorbed on various metal surfaces have been simulated using Fresnel equations for a stratified three-phase model for the interface, assuming each constituent retains its bulk optical properties after the interface is formed, i.e. total neglect of adsorbate-substrate interactions. Results obtained for ultrathin films (1 nm) of squarylium, meso-tetrakis-(phenylporphyrin), and metal-free phthalocyanine in contact with Au, Pt, and the basal plane of highly oriented pyrolytic graphite have shown that the shape of ΔR/R vs λ for p-polarized light depends quite markedly on the angle of incidence and the optical constants of the substrate. At normal incidence, the reflectance spectra displayed peaks similar to those observed in the conventional absorption spectra of the materials. As the angle was increased, however, the position of these features not only shifted, but, in certain cases, new bands emerged, which could not be readily ascribed to the overlayer. This indicates that under non-normal incidence conditions, no conclusions regarding the physical state of adsorbed monolayers on metal substrates can be drawn on the basis of a cursory examination of ΔR/R vs λ curves in this spectral region. These purely optical effects may also account for features observed in the electroreflectance spectra of monolayers irreversibly adsorbed on electrode surfaces, in which case the reflectivity of the substrate is modified by its state of charge.