MANY-BODY CALCULATION OF THE XPS CORE HOLE SPECTRA OF NIN2

The XPS core hole spectra of the two inequivalent N atoms of NiN2 are calculated by the ab initio 2h1p and the 2h2p/3h2p CI methods by using an extended basis set. The calculations give a reasonably good description of the spectral features of the newly separated high resolution XPS core hole spectra of the two inequivalent nitrogen atoms of N2 on a Ni(100) (c(2x2)) surface, such as the main line satellite line energy separations, intensity ratios, and the N1s peak splitting. The calculations show that the peak splitting is indeed due to the two inequivalent nitrogen atoms and not due to charge transfer (CT) screening from different substrate bands. It is shown also that the inner nitrogen hole state is more weakly coupled with the metal atom than the outer nitrogen one. This results in a larger binding energy and absence of the 2.1 eV satellite in the inner nitrogen atom spectrum. The 2.1 eV satellite is interpreted as the pi-CT shakeup satellite and not as the CT screened ionic state resulting from the different spin couplings. For the weakly coupled N2/Ni system where the pi bonding is weaker, the sigma to sigma* excitation associated with local metal configurational changes becomes much more significant for the screening of the core hole than for the strongly coupled CO/Ni system.