Experimental assignment of many-electron excitations in the photo-ionization of NiO

The absorption of a photon and the emission of an electron is not a simple, two-particle process. The complicated many-electron features observed during core photo-ionization can therefore reveal many of the hidden secrets about the ground and excited-state electronic structures of a material. Careful analysis of the photon-energy dependence of the Ni KLL Auger de-excitation spectra at and above the Ni 1s photo-ionization threshold has identified the satellite structure that appears in both the photo-electron emission and the x-ray absorption spectra of NiO as Ni metal 3d eg -> Ni metal 3d eg and O ligand 2p eg -> Ni metal 3d eg charge-transfer excitations, respectively. These assignments elucidate the conflicting theoretical predictions of the last five decades in addition to other anomalous effects in the spectroscopy of this unique material.