Spatial electron distribution of CO adsorbed on Ni(100) and Ni(111) surfaces probed by metastable impact electron spectroscopy

Electron emission spectra obtained by thermal collisions of He*(2 S-3) meta stable atoms with CO on Ni(100) in the c(2x2) structure and on Ni(111) in t he c(4x2) structure were measured to probe directly the spatial electron di stribution. For a systematic comparison, the metastable spectra of free CO, condensed CO on Ni(111), and gaseous Cr(CO)(6) were also measured under th e same beam conditions. Our data showed that the relative ionization cross sections for the CO 4 sigma-, 1 pi-, and 5 sigma -derived states depend dra stically on the molecular orientation of CO with respect to the metastable beam, reflecting the local electron density of CO in the impact region. Mor eover, it was found that the 4 sigma- and 5 sigma- derived states of CO at hollow sites on Ni(111) are strongly modified in space by mixing with each other, where considerable charge transfer occurs from the C site to the O s ite in the 5 sigma -derived state and in the opposite way in the 4 sigma -d erived state. In contrast, such a strong charge redistribution was not seen in the cases of terminal CO on Ni(100) and Cr(CO)(6). These findings were in good accordance with the crystal orbital overlap population obtained by density functional theory through a generalized gradient approximation. (C) 2001 American Institute of Physics.