THEORY OF ALKALI ADSORPTION ON SIMPLE METAL-SURFACES

The electronic structure of alkali adlayers on metal surfaces is studied by first-principles calculations as a function of coverage (theta). Hexagonal Na layers with varying lattice constants are used as adlayers, and the substrate is represented by the semi-infinite jellium. The results obtained show that the classical Gurney model is insufficient to describe the Na-jellium bonding: despite the large potential lowering in the vacuum, the density of states in a Na sphere is rather insensitive to theta. The bonding-antibonding boundary regarding the Na-jellium bonding coincides with the Fermi level at the lowest theta, implying that the covalency in the Na-jellium bond becomes the largest at the lowest theta. The rapid decrease of the Na-induced dipole moment is caused mainly by the direct Na-Na interaction due to the orbital overlap rather than by indirect dipole interaction.