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 $3d{e}_g\to \mathrm{Ni}$ metal $3d{e}_g$ and O ligand $2p{e}_g\to \mathrm{Ni}$ metal $3d{e}_g$ 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.