Resonant Inelastic X-Ray Scattering Spectroscopy
The electronic states that give rise to the edge of an absorption spectrum are resonantly excited states that subsequently decay. The energy that is released in the decay process can be carried either by an electron that is promoted into the continuum (resonant Auger effect) or by a photon. A radiative decay after resonant excitation is in the literature referred to as resonant inelastic X-ray scattering (RIXS), resonant X-ray emission (RXES)or resonant X-ray Raman spectroscopy (RXRS). The combination of resonant excitation with emission detection bears some interesting physical phenomena such as interference effects, line narrowing and Raman-Stokes line shifts.[1-2]
RIXS can be used to study electronic excitations at energies much lower than the incident hard X-ray energy. This is due to the fact that the energy transfer - defined as the difference between incident and emitted energy - describes the electronic excitations. The energy transfer is equivalent to the final state energy of the excited electron configuration. It can be as low as a few meV or as high as several keV. In the former, the spectroscopy is sensitive to phonon excitations, very weak electron-electron or collective excitations. Final state energies of a few eV lie in an energy range that is equivalent to optical excitations directly probing the valence orbitals (but element-selectively). Higher final state energies of more the a few tens of eV contain a core hole in the final state.
] A. Kotani and S. Shin, Resonant inelastic X-ray scattering spectra for electrons in solids. Reviews of Modern Physics, 2001. 73(1): p. 203-46.
 F. Gel'mukhanov and H. Ågren, Resonant X-ray Raman scattering. Physics Reports-Review Section of Physics Letters, 1999. 312(3-6): p. 91-330.