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Abstract

We present a consistent assignment of the electronic excitations in the electron-energy-loss spectra (EELS) of free and chemisorbed CO. The experimental data (for excitations up to 15 eV) show that for a variety of molecular environments (i.e., free, physisorbed, weakly or strongly chemisorbed) the electronic excitation energies change only slightly. New EELS experimental results for the CO/Fe(110) system are also presented. In order to gain some understanding as to the possible origin of this relative constancy in the excitation energies we have carried out a series of ab initio calculations on the ground state and various excited states of CO and NiCO. In the calculations, the generalized valence-bond configuration-interaction method has been employed. On the basis of these calculations it is possible to suggest that when CO is chemisorbed on a metal surface the bond energies of the surface complex for various excited states (arising from 4σ→2π and 5σ→2π excitations) are similar to the bond energy of metal-CO in the ground state. As a consequence, the excitation energies would be expected to be similar for the gas-phase molecule and the chemisorbed molecule, consistent with the experimental findings. Calculations are presented also for a charge-transfer excitation from the metal to the molecule. The concepts used for the valence excitations apply as well for the observed shifts of core-to-bound (C 1s→2π, O 1s→2π) excitations of adsorbed CO, with respect to gas-phase CO. It is suggested that upon chemisorption the relatively large singlet-triplet splitting of the core-to-bound excited states observed in the electron-energy-loss spectra of CO in the gas phase decreases due to interaction with the metal.