Charge transfer excitations in VUV and soft X-ray resonant scattering spectroscopies

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.