On the interpretation of the photoelectron spectrum of CO chemisorbed on Ni

A LCAO-SCF calculation on the NiCO molecule is reported. In addition to the ionization potentials the vibrational structure due to ionization is calculated. The computed ionization potentials are compared with those of recent semiempirical calculations on the CO — Ni system and with the photoelectron spectra of CO chemisorbed on nickel surfaces. We produce further evidence that the first peak in the photoelectron spectrum relates to the 5σ and 1π orbitals of CO and that the peak at higher ionization energy represents the 4σ orbital.