Metal surfaces in interaction with molecular adsorbates

Abstract

For a finite-step barrier model of a metal surface, it is shown that structure in the local density of states, around the bulk density of states, is induced deep into the metal. In a non-free-electron metal, it is stressed that the range over which such structure persists into the bulk metal depends on the shape of the Fermi surface. A summary is then given of the way in which chemical bonds are affected by a metal surface in physisorbed and chemisorbed regimes. A bond parallel to a planar metal surface is weakened and lengthened. Polyatomic molecules are discussed, with particular reference to H2O on the (0001) surface of Ru metal. The principle of maximum overlap predicts the center geometry for the O atom, not the overhead site. The spillout of electron density affects the angle of H2O and near the united atom it opens up the angle a little compared with the free space value for a given O—H length. Finally, the linear response function of the infinite-step barrier model, plus a model potential designed to simulate a flat molecule like benzene adsorbed parallel to the surface, is used to calculate the charge density displaced in the metal. Near the surface, this displaced electron density, along an axis through the center of the benzene ring and perpendicular to it, is shown to be sensitive to the dimensions of the adsorbed molecule. It would therefore be of interest if field gradients due to the displaced charge could be measured.