When an atom in a catalyst is ionized, for instance by the impact of high-energy electrons or photons, an electron from an inner shell is ejected, creating a core-level vacancy. An electron from a higher energy level then fills this vacancy, and the energy released in this process can either be emitted as an X-ray photon or transferred to another electron, causing its ejection. The latter process is known as the Auger effect and the ejected electron is termed an Auger electron.
