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Why does the kinetic energy of an Auger electron not depend on the energy of the incoming beam?

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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 kinetic energy of an Auger electron is independent of the energy of the incoming beam because the Auger process is a secondary relaxation mechanism rather than a direct result of the initial ionization.1

In surface analysis, the primary ionization (caused by an incident photon or electron) creates an inner-shell vacancy, leaving the atom in an excited state. The atom subsequently relaxes to a more stable state when an electron from a higher energy shell drops down to fill that vacancy. The energy released by this transition is then transferred to a second electron—the Auger electron—which is ejected from the atom.123

Because the kinetic energy of the emitted Auger electron is determined solely by the difference in the binding energies of the atomic shells involved in the transition, it remains constant regardless of the energy used to create the initial vacancy. In contrast, techniques like X-ray photoelectron spectroscopy (XPS) measure electrons whose kinetic energy is directly dependent on the energy of the incident radiation.1