Ethylene on Cu(110) and Ni(110):: electronic structure and bonding derived from X-ray spectroscopy and theory

The bonding of ethylene to Cu(110) and Ni(110) is analyzed in detail using symmetry-resolved X-ray absorption (XAS) and emission (XES) spectroscopies in conjunction with density functional theory (DFT) calculations of geometric structure and spectra. XES, which probes the occupied valence states, reveals the formation of bonding and non-bonding orbitals of pi-3d as well as pi*-3d character. Additional mixing of sigma and pi states indicates rehybridization upon adsorption. The anti-bonding pi-3d and pi*-3d combinations are unoccupied and seen in XAS. A lower intensity of the pi* transition for Ni is evidence of larger pi* occupancy upon bonding. The position of the sigma* shape-resonance indicates a 0.02 Angstrom longer C-C bond on Ni than on Cu, in good agreement with the DFT structure optimizations. The XE spectra are well-reproduced both by specific spectrum calculations based on cluster models and by the carbon p-density of states calculated using periodic boundary conditions. The contribution of both pi and pi* levels to the new, surface-induced occupied states close to the Fermi-level tends support to the traditional Dewar-Chatt-Duncan son picture of the bonding. Theoretical charge-density difference plots support an alternative view of ethylene bonding in terms of the specific involvement of the excited molecular triplet state. Based on the variation in XE intensities the main difference between ethylene bonding to Cu and Ni is found to be an about two times larger occupancy of the pi* orbital upon chemisorption on the transition metal, which comes along with C-C bond elongation and stronger sigma-pi rehybridization. (C) 2004 Elsevier B.V. All rights reserved.