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4.1.3 Auger spectroscopy
Impact of electrons with a solid causes ionization of atomic energy levels of atoms in the solid. An ionized atom can relax by the ejection either of an X-ray photon or of an Auger electron; the latter process is far more probable at low primary energies, e.g. less than 10 keV. In the Auger process the ionized level, of binding energy EA, is filled by an electron from an outer level, of binding energy EB, and the excess energy (EA − EB) is given to another electron either in the same level EB or in a still more shallow one of binding energy Ec. The energy EABC of the Auger transition ABC, i.e. the kinetic energy of the ejected electron, is then
where E′C is primed to show that the binding energy of the electron in C has changed due to the fact that the electron is being ejected from an already ionized atom. To a good approximation for most purposes, the Auger energy EABC may be estimated from the Chung and Jenkins’ (1970) expression, for an atom of atomic number Z,
where EB(Z + 1), EC(Z + 1) are the binding energies of electrons in the same levels in the next element up the Periodic Table. Since equation (2) can be used in conjunction with tables of atomic energy levels to calculate the expected energies of Auger transitions, analysis of the observed Auger spectrum from a solid enables its atomic composition to be established.
In electron-excited Auger spectroscopy, the range of energies normally used is 1000–10 000 eV, and the energies of the resultant Auger electrons are typically 20–1000 eV. At these low energies, the inelastic mean free paths of electrons in solids (see section 4.5.2) are very short, and therefore the Auger electrons must originate at, or very close to, the true surface if they are to escape and be observed externally. Auger spectroscopy is thus very surface-specific.
Auger electrons may also be produced by initial excitation with X-rays, ions, and other particles.
The table sets out the experimentally observed energies of the most intense Auger transitions of the more common elements in solid elemental or compound form. Allocation of the transitions has been made on the basis of equation (2), using the tables of binding energies published by Bearden and Burr (1967).
For more comprehensive reading about Auger spectroscopy see the books by Briggs and Seah (1990) and by Rivière (1990).
J. A. Bearden and A. F. Burr (1967) Rev. Mod. Phys., 39,
Energies of the major Auger lines for the more common elements
The symbols K, L, M, N, and O are in the usual X-ray notation for atomic energy levels; the symbol V refers to the valence band of the solid.
J.C. Rivière/M.P. Seah
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