The theory of resonant inelastic X-ray scattering on valence excitations

The coupling between resonant inelastic X-ray scattering (RIXS) and valence excitations is discussed. The concepts of direct and indirect RIXS are described setting the framework for the interpretation of the RIXS cross section. The various mechanism how RIXS couples to dd-transitions, (bi)magnon excitations and phonons are discussed. It is shown how RIXS couples to the dynamic structure factor at the K-edge. At the L-edge, RIXS can be expressed in terms of effective transition operators drawing parallels to optical spectroscopy. Furthermore, selection rules in RIXS are discussed. Finally, the promise of RIXS in the future is discussed, in particular, in connection with X-ray free-electron lasers.