Field and Photofield Emission Spectroscopy of Silver and Sodium Adsorbed on (100) and (110) Facets of Tungsten.

Silver and sodium adatoms on W(100) and W(110) facets of microscopic size have been studied in field and photofield emission. New and interesting findings are reported concerning the growth and the electronic structure of silver and sodium overlayers on tungsten. On (100), silver adatoms form a homogeneous overlayer up to an exposure of 3 monolayers (ML), while the (110) facet remains free of silver up to a threshold exposure of close to 1 ML. At threshold, silver adatoms from the surrounding facets invade the W(110) facet to form a stable overlayer about 1 ML thick. A second monolayer of silver forms on (110), but above 2 ML exposure the adatoms migrate to the surrounding facets. Sodium adatoms on the (100) facet are apparently only weakly bound to the substrate, as the field emission pattern is unstable in appearance, and the applied electric field dramatically affects the overlayer configuration. On the (110) facet, a two-dimensional island of sodium grows on top of the first monolayer, and persists up to 2 ML exposure. Differences in the adsorbate configurations between Ag and Na are interpreted in terms of the atomic diameters of the adatoms relative to the lattice spacings of close packed W(110) and open W(100). The Swanson hump in the surface density of states on W(100) is quenched by less than 0.5 ML of silver atoms, and silver-induced structures on W(100) are predominantly determined by the substrate. The appearance of silver -induced features in the surface density of states on (110), at 1 ML exposure, marks the formation of a two-dimensional band of silver coupled to the (110) substrate. The electronic structures of double overlayers of silver are similar on the two facets. The surface electronic structures of Na/(100) and Na/W(110) are similar, and are closely related to the valence electronic levels of atomic sodium. Self-consistent calculations of the surface densities of states of the metal-adsorbate-vacuum complex are shown to be generally consistent with the experimental data.