Evidence for longitudinal waves in electroreflectance spectroscopy of silver

Electroreflectance spectra of single crystal Ag electrodes have been recorded for various degrees of surface charging. The experimental results obtained with p-polarized light at nonnormal incidence are compared with model calculations for a stratified three layer system. The characteristic spectral features in the vicinity of the volume plasma frequency can be explained only by inclusion of longitudinal plasma waves in the surface layer as well as in the bulk of Ag. By comparison of theory and experiment it is deduced that, at the potential of zero charge, the conduction electron density in the metal surface region is considerably lower than the respective bulk value.