Electronic structure of CO adsorbed on small Cu clusters: Theoretical study on excited states

Ab initio self-consistent-field configuration-interaction calculations are carried out for CO absorbed on small Cu clusters (${\mathrm{Cu}}_{\mathrm{n}}$CO, n=1\char21{}5). The primary purpose of this paper is to discuss the electron-energy-loss spectra in terms of the low-lying intra-CO valence excited states and charge-transfer (CT) excitations of a 3d electron from Cu to CO. The intra-CO 5\ensuremath{\sigma}\ensuremath{\rightarrow}2\ensuremath{\pi}, 1\ensuremath{\pi}\ensuremath{\rightarrow}2\ensuremath{\pi}, and CT transitions contribute to the lower two bands of 6\char21{}7 and 8\char21{}10 eV. The 4\ensuremath{\sigma}\ensuremath{\rightarrow}2\ensuremath{\pi} transition contributes to the band around 12 eV and/or the band lying at 13\char21{}14 eV.