Analysis of CO-Cu bonding by observation of interface states on Cu/Ru bimetallic surfaces.

Detailed analysis of the properties of interface states observable when monolayer levels of Cu are adsorbed on Ru(0001) reveals that these states are ideally suited for studies of the nature of adsorbate-metal bonding. Angle-resolved photoemission spectra are presented showing that the shallow interface states of the Cu/Ru surface shift to larger binding energies, while the deeper states decrease in intensity without an appreciable shift. Analysis of the symmetries of the interface states and of their positions relative to the bulk bands of Ru leads to the conclusion that the CO-Cu interaction does not involve any appreciable hybridization with metal d-orbitals, but rather the formation of a CO(5sigma) - Cu(4s) bond as a result of the polarization of the Cu(4s) orbitals away from the CO. The fact that CO bonds more strongly to Cu/Ru than to pure Cu is attributed to the enhanced polarizability of Cu when it lies in proximity to Ru.