NEW INTERPRETATION OF THE XPS CORE HOLE SPECTRA OF CO ON A NI(100) SURFACE

The XPS core hole spectra of NiCO are calculated by the ab initio 2h1p (two hole one particle) and 2h-2p/3h2p CI method by using an extended basis set. The main-line/satellite-line intensity ratios and energy separations of the new high-resolution XPS core hole spectra of the CO/Ni(100) system are well reproduced by the 3h2p CI method. The calculations show that the main-line state is the 1h state closer to the Koopmans state, rather than the 2h1p charge transfer (CT) shake-down state. The newly resolved 2.1 eV carbon satellite is well reproduced by the present calculation. The corresponding oxygen satellite intensity is very small, in accord with experiment. This satellite is the pi-2h1p CT shake-up state (with respect to the 1h main-line state). The 5.5 eV satellites are also obtained by the present scheme and interpreted as the sigma-CT shake-up state (with respect to the main-line state), rather than the pi-shake-up state (with respect to the 2h1p CT shake-down state). The 9.5 eV carbon satellite and 8.5 eV oxygen satellite are interpreted as the pi-CT shake-up state. For the adsorbate in the smaller shake-up energy region, the spectral features are predominantly determined by the metal-ligand CT excitations. The importance of the higher-order valence excitations in the lower shake-up energy region is emphasized.