  The utility of resonant scattering spectroscopies for identifying electronic
symmetries and density distributions changes dramatically as a function of
photon energy. In the hard X-ray regime, strong core hole monopole potentials
tend to produce X-ray absorption features with well defined electron number on
the scattering site. By contrast, in the vacuum ultraviolet (VUV), resonant
scattering from Mott insulators tends to reveal spectra that are characteristic
of only the nominal valence, and are insensitive to deviations from nominal
valence brought on by metal-ligand hybridization. Here, atomic multiplet
simulations are used to investigate the interplay of monopolar and mulitpolar
Coulomb interactions in the VUV and soft X-ray regimes, to identify how charge
transfer thresholds and other signatures of mixed valence can manifest in this
low photon energy regime. The study focuses on the Mott insulator NiO as a well
characterized model system, and extrapolates interactions into non-physical
regimes to identify principles that shape the spectral features.
