  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.
