Electron emission spectra caused by thermal collisions of He* (23S) metastable atoms with Ni(100)c(2×2)-CO and Ni(111)c(4×2)-CO were measured to probe the local electron distribution. Our data showed that the 4σ- and 5σ-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 atom to the O atom in the 5σ-derived state and in the opposite way in the 4σ-derived state. In contrast, such a heavy charge redistribution was not seen in the case of terminal CO on Ni(100). These findings were in good accordance with the crystal orbital overlap population obtained by density functional theory within the generalized gradient approximation.
