Surface and Interface States on Silver Single Crystals Studied Using Electroreflectance.

Intrinsic surface states on silver single crystal (110) and (100) faces, their interactions with various adsorbates, and interface states were studied in the electrochemical environment by using electroreflectance. Electroreflectance is an electric field modulation of the specularly reflected light. Because of the small dimension of the metal/electrolyte interface (~10A), a strong electric field (~10^7 V/cm) is obtained at the interface by applying a moderate potential (~1 V). The high electric field is utilized to modulate the optical and electronic properties of the interface. The surface states, localized at the surface, are very sensitive to the change of the surface potential and surface adsorbates. Therefore, the surface states were chosen to study the metal/electrolyte interface. The surface states were found to be at 1.91 eV on Ag(110) surface and at 3.3 eV on Ag(100) surface above the Fermi level for zero electric field. On Ag(100) surface, an additional surface state was found at around 1 eV above the Fermi level. As expected, all surface states were found to be very sensitive to the change of the potential difference across the interface. Careful examination of the potential dependence of these states reveals that the potential distribution in the double layer does not follow the classical theory. Adsorption study of chloride shows that the lower energy surface state on Ag(100) is involved in making a chemisorption bond between chloride and silver, while the higher energy surface state on Ag(100) and the surface state on Ag(110) are not. A new spectral feature at around 3.5 eV was observed by adsorption of the iodide. This feature is attributed to the charge transfer from the filled iodide induced state to the empty silver state at the Fermi level. All surface state features were quenched completely with adsorbed iodide, showing that the surface is perturbed strongly. In the case of pyridine adsorption, the interaction of the surface state is found to be closely related to the orientation of the pyridine on the silver surface.