X-ray Absorption Spectroscopy (XAS)
In X-ray absorption spectroscopy (XAS) the energy of the absorbed photon raises an electron from a deeply bound state into unoccupied bound states or it gains enough energy to escape the atom. The absorption spectrum thus contains detailed information about the density of empty states and allows conclusions about coordination, oxidation state and many more information about the local structure. If the energy of the the photon is sufficient to overcome the binding potential the electron the absorption probability is influenced by a electron scattering process from the local environment of the surrounding atoms. This technique called EXAFS and can be used to determine the local structure around the absorbing atoms.
X-ray Emission Spectroscopy
The electrons filling the core hole that is generated during the absorption process emit X-ray photons whose energy can be analyzed. The fine structure of this X-ray emission spectroscopy (XES) can give detailed information on the density of filled states and can be used due to the influence of spin-orbit coupling to track the oxidation and total spin state of the emitting atom.