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How do Auger electrons form after an atom is ionized?

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Moon rock crystals — surface analysis A scanning-electron-microscope photograph of pyroxene and plagioclase crystals (the long and the short crystals, respectively) that grew in a cavity in a fragment of Moon rock gathered during the Apollo 14 mission.

The Auger effect is a spontaneous process that occurs when an atom, having been ionized by the removal of an electron from an inner shell, returns to a more stable state.1

When an inner-shell vacancy is created—typically through electron bombardment or X-ray irradiation—an electron from a higher-energy outer shell will drop down to fill that vacancy. This transition releases a specific amount of energy. Instead of emitting this energy as an X-ray photon, the atom may dissipate it by ejecting a second electron from one of its shells. This ejected electron is known as an Auger electron.12

As a result of this emission, the residual atom is left with two electron vacancies. In the context of surface analysis, this process is highly valuable; by measuring the kinetic energies of these emitted Auger electrons, scientists can identify the elemental composition of a material's surface.123