Molecular-orbital cluster-model study of the core-level spectrum of CO adsorbed on copper

We have used ab initio self-consistent-field wave functions for a Cu${}_5{}^{}{}_{}{}^{}\mathrm{CO}$ cluster to model the interaction of CO with a Cu surface and to study the x-ray photoemission (XPS) from CO core levels. In order to justify the use of this cluster model of the chemisorption of CO on Cu, we show that we obtain reasonable values for the ground-state Cu-CO bond distance and bond strength and accurate values for the CO core-level ionization potentials. An extensive analysis of the initial-state chemical bonding and the final-state relaxation processes is given. We show that two types of final ${\mathrm{C}}_{1s}$ or ${\mathrm{O}}_{1s}$ core hole states exist with comparable photoionization intensities. The lowest state is a shakedown state in which a Cu $4sp$ valence electron is transferred to the CO $2{\pi }^{*}$ level effectively screening the core hole. The higher-lying final state closely resembles a "normal" one-hole core ion in which the metal electrons participate in the screening in only a very limited way. Our analysis shows that the intensity distribution between these two states is closely related to the extent of $2{\pi }^{*}$ backbonding in the unionized ground state of the system. We consider also the effects of spin coupling of the core hole to the $2{\pi }^{*}$ electron for the shakedown states. The existence of these two types of relaxed final states, shakedown and normal, is responsible for the broad core-level peaks observed in XPS spectra. This conclusion, based on a molecular-orbital analysis is similar to that reached by Schönhammer and Gunnarsson who used a parametrized Anderson-type Hamiltonian to describe the CO-Cu interaction.