A type of X-ray spectroscopy that is used to investigate the electronic structure of molecules and solids. When an X-ray impinges on an atom and excites a core electron to an excited state, the absorption of the X-ray gives information about the excited state. The creation of a hole among the core electrons in this way causes an electron from a higher energy level than the core electron to fill the hole, causing, in turn, the emission of a photon. This gives information about the electronic states that were occupied before the X-ray impinged on the atom.