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

Electroreflectance spectroscopy in the visible and near-UV has been em ployed to study the influence of adsorbed pyridine on the surface reco nstruction 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 optic ally 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 molecul e from a flat pi-bonded to a vertical N-bonded surface coordination. P yridine adsorbs flat lying on the reconstructed surface, which is stab le at negative surface charges, while on the unreconstructed, more ope n surface at positive charges the pyridine molecules assume a vertical , N-bonded position. Optical transitions involving surface states whic h display a strong dependence on the electrode potential (charge densi ty) due to a Stark shift, were studied in the absence and presence of pyridine. Adsorption of pyridine causes a broadening and a gradual dec rease of the surface state spectral features which suggests that the s urface states undergo an adsorbate-induced level broadening, but are n ot quenched completely.