Experimental assignment of many-electron excitations in the photoionization 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 photoionization 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 deexcitation spectra at and above the Ni 1s photoionization threshold has identified the satellite structure that appears in both the photoelectron emission and the x-ray absorption spectra of NiO as Ni metal 3deg→Ni metal 3deg and O ligand 2peg→Ni metal 3deg 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.