Toward the Interpretation of Electroreflectance Spectral Profiles: Hemin Adsorbed on an HOPG Electrode Revisited

Abstract

The features of the potential-modulated UV-visible reflectance (electroreflectance) spectrum at an electrode/solution interface are discussed by comparing experimental and simulated spectra. At a basal plane of a highly oriented pyrolytic graphite (HOPG) electrode covered with a molecular layer of hemin in 0.1 M Na₂B₄O₇ aqueous solution, the electroreflectance signal was confirmed to be proportional to the amount of adsorbed hemin interconverted between oxidized and reduced forms. The electroreflectance spectrum in response to p-polarized incident light depended little on the incident angle, and the spectral profile was different from the difference absorption spectrum between oxidized and reduced hemin in solution phase. The spectral feature of the simulated electroreflectance spectrum with the use of Fresnel equations for a stratified three-phase optical model with a thin-layer approximation was markedly different from the experimental spectrum in regard to spectral profile and incident angle dependence. These results may suggest that refinement of the optical model to predict the reflectance spectrum at an electrode surface covered with a molecular layer is necessary for the interpretation of electroreflectance spectral profiles.