ELECTROREFLECTANCE SPECTROSCOPY OF AU(100) COVERED BY ADSORBED PYRIDINE MOLECULES

Electroreflectance spectroscopy in the visible and near-UV has been employed to study the influence of adsorbed pyridine on the surface reconstruction and the optical properties of Au(100). The pyridine-induced structural transition of the Au(100) electrode from a reconstructed (5 x 20) to an unreconstructed (1 x 1) surface has been monitored optically for various pyridine concentrations. It has been found that this surface structural transition coincides with a phase transition in the pyridine adlayer, which corresponds to a reorientation of the molecule from a flat pi-bonded to a vertical N-bonded surface coordination. Pyridine adsorbs flat lying on the reconstructed surface, which is stable at negative surface charges, while on the unreconstructed, more open surface at positive charges the pyridine molecules assume a vertical, N-bonded position. Optical transitions involving surface states which display a strong dependence on the electrode potential (charge density) due to a Stark shift, were studied in the absence and presence of pyridine. Adsorption of pyridine causes a broadening and a gradual decrease of the surface state spectral features which suggests that the surface states undergo an adsorbate-induced level broadening, but are not quenched completely.