Here is an overview of methods which we use in our investigations

X-ray Spectroscopy

X-ray absorption near edge spectroscopy (XANES) is a widely used technique to reveal the local and electronic structure of matter. When the X-rays hit the sample, an electron is promoted from the ground state level to the first unoccupied state. However, the core hole that is created by that process is very unstable and very quickly is filled by an electron from the other levels. The X-ray photons emitted during that process are measured by X-ray emission spectroscopy (XES). Therefore, both techniques provide complementary information about the occupied and unoccupied states. The X-ray absorption and emission lines, detectable in the hard X-ray region for the different elements and their life-time broadenings, can be found in the literature. Standard XES is recorded with non-resonant excitations. The XES process recorded with resonant excitations is known as resonant X-ray emission spectroscopy (RXES) and resonant inelastic X-ray scattering (RIXS).

High energy resolution fluorescence detection (HERFD), RIXS, XES

Schematic representation of the XANES, HERFD, XES and RIXS measurements at the U M4 edge for UO2 and electronic transitions of those processes. RXIS data are shown as contour maps in a plane of incident (Ei) and transferred photon energies (ET), where the vertical axis represents the energy difference between the incident and the emitted energies (Ee). Variations of the color in the plot relate to different scattering intensities. The HERFD spectrum corresponds to a diagonal cut through the RIXS planes at the maximum of the emission line.

Good books about it:

F. de Groot and A. Kotani, Core Level Spectroscopy of Solids (CRC Press, 2008)

A. Meisel, G. Leonhardt, and R. Szargan, Springer Ser. Chem. Phys. 37, (1989)

P. Glatzel and A. Juhin, in Local Struct. Characterisation (John Wiley and Sons, Ltd, Chichester, UK, 2013), pp. 89–171

J.A. Van Bokhoven and C. Lamberti, editors , X-Ray Absorption and X-Ray Emission Spectroscopy (John Wiley and Sons, Ltd, Chichester, UK, 2016)