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One of the two principal processes for the relaxation of an inner-shell electron vacancy in an excited or ionized atom. The Auger effect is a two-electron process in which an electron makes a discrete transition from a less bound shell to the vacant, but more tightly bound, electron shell. The energy gained in this process is transferred, via the electrostatic interaction, to another bound electron which then escapes from the atom. This outgoing electron is referred to as an Auger electron and is labeled by letters corresponding to the atomic shells involved in the process. For example, a KLILIII Auger electron corresponds to a process in which an LI electron makes a transition to the K shell and the energy is transferred to an LIII electron (illus. a). By the conservation of energy, the Auger electron kinetic energy E is given by E = E(K) - E(LI) - E(LIII) where E(K,L) is the binding energy of the various electron shells. Since the energy levels of atoms are discrete and well understood, the Auger energy is a signature of the emitting atom. See Electron configuration, Energy level (quantum mechanics)
The other principal process for the filling of an inner-shell hole is a radiative one in which the transition energy is carried off by a photon (illus. b). Inner-shell vacancies in elements with large atomic number correspond to large transition energies and usually decay by such radiative processes; vacancies in elements with low atomic number or outer-shell vacancies with low transition energies decay primarily by Auger processes.