CONFIGURATION AND ELECTRONIC-STRUCTURE OF SILVER OVERLAYERS ON MICROSCOPIC (110) FACETS OF TUNGSTEN

A field emission study of a tungsten tip exposed to a flux of silver s hows that at room temperature a microscopic W(110) facet remains free of adsorbate atoms up to a threshold exposure of close to one monolaye r. At threshold, silver atoms from the surroundings facets invade the (110) facet, forming a stable two-dimensional overlayer. Total emissio n current and work function data indicate that the overlayer greatly e nhances the surface density of electron states in the vicinity of the Fermi level. This finding is consistent with electron spectroscopy dat a, which reveal an adsorbate-induced contribution to the surface densi ty of states that extends to about 0.2 eV below the Fermi level. At tw o monolayers exposure a second overlayer forms, with a characteristic surface density of states. However, a third overlayer is found to be u nstable, as atoms deposited on top of the second overlayer tend to mig rate to the surrounding facets.