Valence-band photoemission and optical absorption in nickel compounds

Photoemission, optical-absorption, and isochromat spectra of NiO and ${\mathrm{NiCl}}_2$ are studied theoretically by the consideration of configuration interactions within the metal-ligand cluster. It is shown that the satellites in the valence-band photoemission spectra contain significant $d^7$ final-state components produced by photoemission of a $d$ electron from the largely $d^8$-like ground state and that final states giving the main lines are predominantly $d^8$-like resulting from ligand $\to 3d$ charge-transfer transitions following the $d$-electron emission. This identification differs markedly from the traditional one, according to which the main lines are due to $d^7$ final states and the satellites are produced by ligand $\to 3d$ shakeup transitions. The crystal-field splitting and the apparent reduction of Racah parameters are shown to be due to hybridization between different configurations. The resonance enhancement of the satellites rather than the main lines at the $3p\to 3d$ photoabsorption threshold is attributed partly to covalency and partly to the small number of $3d$ holes in the nickel compounds as compared to other $3d$ transition-metal compounds. Excitation energies for ligand $p\to \mathrm{Ni} 3d$ charge-transfer optical absorption are calculated and it is shown that the fundamental absorption edge of NiO at ~4 eV is not due to the $p\to d$ charge-transfer transitions. Instead, $d\to d$ charge-transfer transitions are proposed as the origin of the NiO fundamental edge. Energy levels involved in the intra-atomic $d\to d$ optical absorption are also calculated by the configurationinteraction approach and good agreement with experiment and energy levels calculated by the ligand-field theory is obtained. Finally the isochromat spectrum of NiO is discussed, based on the same approach.