Theory of the alkali-metal chemisorption on metal surfaces

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 {δ}{ρ}(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 {δ}{ρ}(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}.