Toward the interpretation of electroreflectance spectral profiles: Hemin adsorbed on an HOPG electrode revisited

The features of the potential-modulated UV-visible reflectance (electrorefl ectance) spectrum at an electrode/solution interface are discussed by compa ring experimental and simulated spectra. At a basal plane of a highly orien ted pyrolytic graphite (HOPG) electrode covered with a molecular layer of h emin in 0.1 M Na2B4O7 aqueous solution, the electroreflectance signal was c onfirmed to he proportional to the amount of adsorbed hemin interconverted between oxidized and reduced forms. The electroreflectance spectrum in resp onse to p-polarized incident light depended little on the incident angle, a nd the spectral profile was different from the difference absorption spectr um between oxidized and reduced hemin in solution phase. The spectral featu re of the simulated electroreflectance spectrum with the use of Fresnel equ ations for a stratified three-phase optical model with a thin-layer approxi mation was markedly different from the experimental spectrum in regard to s pectral profile and incident angle dependence. These results may suggest th at refinement of the optical model to predict the reflectance spectrum at a n electrode surface covered with a molecular layer is necessary for the int erpretation of electroreflectance spectral profiles.