When an electron is removed from a core level of an atom, leaving a vacancy, an electron from a higher energy level will fall into the vacancy, resulting in a release of energy. Although sometimes this energy is released in the form of an emitted photon, the energy can also be tranfered to another electron, which is then ejected from the atom. This ejected electron is called an Auger electron (pronounced O-jay where the j is a soft j) and the process by which it is ejected is called the Auger emission process.
Upon ejection the kinetic energy of the Auger electron corresponds to the difference between energy of the initial electronic transition and the ionization energy for the shell from which the Auger electron was ejected. These energy levels depend on the type of atom and the chemical environment in which the atom was located. Auger electron spectroscopy stimulates the emission of Auger electrons by bombarding a sample with either X-rays or energetic electrons and measures the intensity of Auger electrons as a function of the Auger electron energy. The resulting spectra can be used to determine the identity of the emitting atoms and some information about their environment.