Theory of the alkali-metal chemisorption on metal surfaces

Abstract

The electronic structure of the alkali-metal adatom on metal surfaces is studied by a first-principles method as a function of adatom coverage (Θ). We employ ‘‘jellium’’ as a high-density metal substrate to make a continuous change of Θ possible. Although the characteristic variation of the work function with Θ is reproduced well by the present calculation, its mechanism is different from a widely accepted mechanism in which the adatom electronic structure is assumed to change from ionic to neutral with increasing Θ by the depolarization shift. The charge redistribution δρ(r,Θ) that lowers the work function deviates far from the point-charge-transfer model, and the electrostatic potential change at adatom sites due to δρ(r,Θ) depends very little on Θ. Accordingly, the adatom valence density of states shows no downward shift with increasing Θ. The adatom region is essentially neutral, even at low Θ. The bonding-antibonding boundary in the bond-order density of the adatom-substrate bond coincides well with the Fermi level at low Θ, indicating a formation of a metallic bond by the maximum use of bonding states even at low Θ. The close similarity between the calculated bond-order and dipole densities as a function of the one-electron energy implies that the adatom polarization due to the hybridization of adatom and substrate orbitals plays an important role for the adatom dipole and its Θ dependence even at low Θ. The decrease of the adatom dipole is explained by a weakening of the adatom-substrate bonding as well as a significant decrease in the dipole matrix elements with increasing Θ.