What can we learn using X-ray fluorescence and Auger electron coincidence spectroscopy?

X-ray fluorescence and Auger electron coincidence spectroscopy (XAECS) is formulated in a systematic manner by treating the initial ionization (excitation) and subsequent core-hole decays on an equal footing. For this purpose the unitary expansion method is used. XAECS and Auger photoelectron coincidence spectroscopy (APECS) are compared. The unique possibility of the deexcitation electron spectroscopy (DES) measurement in coincidence with the X-ray fluorescence from the resonantly excited core-hole state using the XAECS technique, is discussed, in particular from the viewpoint of the competition between the charge transfer core-hole screening and the core-hole decay.