XANES electronic transitions into unoccupied

In the XANES region of the X-ray absorption spectrum, two physical processes play a significant role. These are electronic transitions into unoccupied electronic states and multiple scattering (MS) resonances. 

XANES Transitions into Unoccupied Electronic States.439... 

The electronic transitions probed by x-ray absorption spectroscopy involve the excitation of a core electron into either unoccupied bound electron states near the Fermi level of the material or at higher energies into the continuum of states producing a photoelectron. These electronic excitations must obey spectroscopic selection rules and thus can provide information about the symmetry of an atom s environment, its oxidation state, and sometimes, with the assistance of comprehensive theoretical calculations, details about the geometry of ligands and other nearby atoms. This information is derived from excitations into bound states and low lying resonances above the Fermi level and is referred to as the x-ray absorption near edge structure (XANES). 

The XANES spectrum for P409 80,140) shows a preedge absorption band at 2148.2 eV, which is assigned to an electron transition from the P(III)-ls orbital into an unoccupied MO with main contributions from P(III) and 0. Above 2150 eV, two broad resonances have been observed, which could be fitted by three Voigt profiles of approximately... 

XANES probes the transition of core electrons into unoccupied electronic states. 

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