ALKALI ADSORBATES ON METAL-SURFACES - OBSERVABLE CONSEQUENCES OF THE IONIC K/CU(100) INTERACTION

We have examined the consequences for several observable properties of the adsorption and chemical bonding of alkali atoms on metal surfaces. The properties considered are vibrational frequencies, core level binding energies, BE's, and work functions. Our results are based on the use of molecular orbital, electronic wavefunctions for cluster models of K/Cu(100). Our principle concern is for the low coverage limit where our cluster model wavefunctions show that the Cu-K bond is dominantly ionic; however, we also consider models for covalently bonded K to represent high coverage adsorption of the alkali. All the properties that we have studied are consistent with a fully ionic alkali adsorbate at low coverage. In particular, we show: (1) the large dynamic dipole moment of ionic K on a metal surface leads to a large observed intensity of the K vibrations for the frustrated translational mode normal to the surface; (2) a small BE shift for the core levels of the surface atoms of the substrate is obtained even for an ideal ionic point charge model to represent the K+ adsorbate; and, (3) the adsorbate BE's are smaller by about 1.5 eV when the Cu-K bond is covalent than when it is ionic. This is shown to be consistent with the observed decrease in the core level BE's of alkali adsorbates as their coverage on the substrate increases. Recent proposals that the interaction between alkali adsorbates and metal surfaces are covalent at all coverages, including the limit of zero coverage, are not supported by the present study.