Remember that the production of Characteristic X-rays is a two-stage process: ionisation followed by relaxation with the production of an X-ray photon. However, this is not the whole story, and there is a competing process, the generation of Auger electrons, that can also lead to stabilization of the ionized atom. In this case, when an electron from an outer shell fills the vacancy in the ionized inner shell an electron is ejected from the same or another outer shell and no X-ray photon is produced. The ejected electron is known as an Auger electron and it has an energy equal to the difference between the ionisation energies of the two shells involved in the initial transition minus the ionisation energy of the shell from which the Auger electron is ejected. So the energy of the Auger electron is also related to the electronic configuration of the atom from which it came and is characteristic of the element concerned.

After inner shell ionisation, the atom may relax by emitting a Characteristic X-ray or an Auger electron. The energy of the Auger electron is related to the electronic configuration of the atom that was ionized by the primary electron beam.

Auger animation

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The fluorescence yield is the relative yield of X-rays to Auger electrons. Elements with low ionisation energies, i.e. the lighter elements (Z<11), have low fluorescence yields. That is, when an inner-shell ionisation occurs it is more likely that an Auger electron will be produced rather than an X-ray photon. The intensities of X-ray peaks for elements of low atomic number are smaller compared to those with a higher fluorescence yield.