Binding and energy-level shifts of carbon monoxide adsorbed on nickel: Model studies

Abstract

Hartree-Fock calculations on linear NiCO and ${\mathrm{Ni}}_{2}$CO clusters have been used to model the local binding of CO upon adsorption on Ni. The Ni-CO bond is determined by a mixing of the $\mathrm{CO}(5\ensuremath{\sigma})$ and $\mathrm{Ni}(3d)$ orbitals; the $\mathrm{Ni}(4s)$ electrons are not directly involved in the bond as can be seen from orbital contour plots. A knowledge of the absolute and relative shifts of molecular ionization potentials from their free-molecule values is important for the interpretation of the photoelectron spectra of chemisorbed species. For our model clusters, we have made a detailed analysis of the origins of the shifts for CO on Ni. The different contributions to the energy-level shifts of the CO-like orbitals show a grouping into core, nonbonding valence, and bonding valence levels. Chemical shifts due to the changed environment of the adsorbed molecule are seen to be important for the initial-state shifts of the nonbonding orbitals. A bonding shift appears to be important only for the CO $5\ensuremath{\sigma}$ level. The relaxation shifts are different among the different groups of levels. Analysis of the calculated and observed shifts gives support to the assignment that the order of the $5\ensuremath{\sigma}$ and $1\ensuremath{\pi}$ levels for CO adsorbed on Ni is the same as for free CO; $5\ensuremath{\sigma}$ less strongly bound than $1\ensuremath{\pi}$. The $\mathrm{O}(1s)$ and $\mathrm{C}(1s)$ core-level shifts indicate that the CO bond is stretched only slightly upon adsorption. The multiplet splitting of the final ionic states is considered and found to be small.