Electronic structure of CO adsorbed on a Cu(111) surface analyzed with molecular cluster models

Using the self-consistent Hartree-Fock-Slater model we have calculated the electronic structure for various ${\mathrm{Cu}}_x{\left(\mathrm{CO}\right)}_y$ clusters symbolizing not only CO bound to "on-top" and "bridge" sites but also some lateral CO-CO interaction on a Cu(111) surface. By comparison with experimental photoemission data we are able to reproduce the observed energies of the occupied CO $4\sigma$, $1\pi$, and $5\sigma$ orbitals as well as the partly occupied $2\pi$ orbital. In our model we assume CO to be adsorbed on "top" sites for coverages less than $\ominus =0.33\mathrm{}$ [$(\sqrt{3}\times \sqrt{3})R30\mathrm{}°$] and on both top and bridge sites for $\ominus >0.33\mathrm{}$. The experimentally observed peak of intensity at the Fermi edge which increases with coverage above 0.33 is in our model explained by the occupation of CO orbitals of the $b_1$ and $b_2$ symmetry types, i.e., "$\pi$" orbitals, degenerate for top positions ($C_{3v}$), split by the change to bridge positions ($C_{2v}$). Our results further indicate that the broadened $5\sigma -1\mathrm{}\pi$ intensity peak at high coverages is a result of CO bound to top and bridge sites.