APS - Appearance Potential Spectroscopy APS is a generic term which involves several techniques such as Auger Electron Appearance Potential Spectroscopy (AEAPS), Soft X-ray Appearance Potential Spectroscopy (SXAPS) and Disappearance Potential Spectroscopy (DAPS). These techniques involve gradually increasing the energy of the exciting beam and detecting the onset of the excitation of a core level. Hence they are complementary to EXAFS and related techniques, but EXAFS is more bulk sensitive.