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 (\ensuremath{\Theta}). We employ ``jellium'' as a high-density metal substrate to make a continuous change of \ensuremath{\Theta} possible. Although the characteristic variation of the work function with \ensuremath{\Theta} 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 \ensuremath{\Theta} by the depolarization shift. The charge redistribution \ensuremath{\delta}\ensuremath{\rho}(r,\ensuremath{\Theta}) that lowers the work function deviates far from the point-charge-transfer model, and the electrostatic potential change at adatom sites due to \ensuremath{\delta}\ensuremath{\rho}(r,\ensuremath{\Theta}) depends very little on \ensuremath{\Theta}. Accordingly, the adatom valence density of states shows no downward shift with increasing \ensuremath{\Theta}. The adatom region is essentially neutral, even at low \ensuremath{\Theta}. The bonding-antibonding boundary in the bond-order density of the adatom-substrate bond coincides well with the Fermi level at low \ensuremath{\Theta}, indicating a formation of a metallic bond by the maximum use of bonding states even at low \ensuremath{\Theta}. 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 \ensuremath{\Theta} dependence even at low \ensuremath{\Theta}. 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 \ensuremath{\Theta}.