Theory of the alkali-metal chemisorption on metal surfaces. II.

Abstract

The electronic structure of alkali-metal adlayers on metal surfaces is studied by first-principles calculations within the local-density-functional theory as a function of coverage (Θ). Hexagonal Na layers with varying lattice constants are used as adlayers, and the substrate is modeled by the semi-infinite jellium with ${\mathit{r}}_{\mathit{s}}$=2, 3, and 4. The results obtained refine upon those in a previous work where the substrate was approximated by a jellium slab [Phys. Rev. B 38, 8006 (1988)]. In spite of the large potential lowering in the vacuum, the density of states in a Na sphere is rather insensitive to Θ, except that the atomiclike resonances at low Θ are broadened with increasing Θ because of the formation of adlayer bands. For all the substrates, the bonding-antibonding boundary with regard to the Na-jellium bonding coincides with the Fermi level at the lowest Θ, which implies that the covalency in the Na-jellium bond and the interatomic polarization term in the Na-induced dipole moment become the largest at the lowest Θ. The rapid decrease of the dipole moment at higher Θ is caused mainly by the direct Na-Na interaction due to the orbital overlap, which leads to a stronger Na-Na bond and simultaneously to a weaker Na-jellium bond, rather than by the indirect dipole-dipole interaction.