AES Theory: The Auger Process

The basic Auger process starts with removal of an inner shell atomic electron to form a vacancy. Several processes are capable of producing the vacancy, but bombardment with an electron beam is the most common. The inner shell vacancy is filled by a second atomic electron from a higher shell. Energy must be simultaneously released. A third electron, the Auger electron, escapes carrying the excess energy in a radiationless process. The process of an excited ion decaying into a doubly charged ion by ejection of an electron is called the Auger process. Alternatively, an X-ray photon removes the energy. For low atomic number elements, the most probable transitions occur when a K-level electron is ejected by the primary beam, an L-level electron drops into the vacancy, and another L-level electron is ejected. Higher atomic number elements have LMM and MNN transitions that are more probable than KLL.

The figure illustrates two competing paths for energy dissipation with titanium as an example. The illustrated LMM Auger electron energy is ~423 eV (E_Auger = E_L2 - E_M4 - E_M3) and the X-ray photon energy is ~457.8 eV (E_hv = E_L2 - E_M4).

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