Ionic and covalent electronic states for K adsorbed on Cu₅ and Cu₂₅ cluster models of the Cu(100) surface

Abstract

The chemisorption of K on the fourfold hollow site of the Cu(100) surface has been theoretically investigated by means of Cu5-K and Cu 25-K cluster models. We have analyzed Hartree-Fock self-consistent field (SCF) wave functions for various electronic states of the two clusters. Four different measures have been used to establish the degree of ionicity of each state: (1) the analysis of the dipole moment curve for the variation of the Cu-K distance; (2) a constrained variation of the SCF orbitals to separate electrostatic, polarization, and charge transfer contributions; (3) the projection of the K valence orbitals onto the cluster wave function to measure the orbital occupancies, and (4) an energetic analysis of the cost and benefit of forming an ionic bond. We found different properties for the two clusters. All the considered electronic states of Cu25-K show large ionic character, suggesting that the bonding of K to a Cu(100) surface is indeed ionic at low coverage. The bonding character of the lowest states of Cu5-K is different, ranging from dominantly ionic to dominantly covalent. This behavior for Cu5-K is related to the small size of the cluster but it can be useful for modeling the transition from ionic to metallic bonding as the coverage of the alkali metal increases. © 1995 American Institute of Physics.