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AES is based on a two‐step mechanism (Figure 5.10a). Collision of an incident electron beam with the surface of a material leads to the emission of an electron from an inner atomic orbital (binding energy = E1) leaving a vacant site that is soon filled by an electron from an outer orbital (binding energy = E2). The energy released in this transition may appear as an X‐ray photon or may be transferred to another electron in an outer orbital (binding energy = E3) which is ejected with kinetic energy E_k given by

(5.12)

The ejected electron is called an Auger electron. The electron ejected from the outer orbital moves through the solid and soon loses its energy through inelastic collisions with bound electrons. On the other hand, if the Auger electron is emitted sufficiently close to the surface of the material, it may escape and be detected using a spectrometer. The number of emitted electrons (or the derivative of the counting rate) is measured and plotted as a function of the kinetic energy of the electron. Since each type of atom has its own characteristic energy levels, the peaks in the observed Auger spectrum can be used to determine the elemental composition of the surface by comparison with standard Auger spectra for the elements.

Figure 5.10 Interaction of incident beam (electrons or X‐rays) with a solid, producing atomic excitation with (a) emission of electrons followed by de‐excitation and emission of Auger electrons or (b) emission of photoelectrons.