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How does Auger electron spectroscopy identify elements on a surface?

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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.

Auger electron spectroscopy (AES) identifies chemical elements on a surface by measuring the kinetic energy of emitted Auger electrons, which are characteristic of the specific atoms from which they originate.12

The process begins when an electron beam strikes the surface, creating a vacancy in an inner electron shell of an atom. As an electron from a higher energy shell drops to fill this vacancy, it releases energy. Instead of emitting an X-ray, this energy is transferred to a secondary electron—the Auger electron—which is then ejected from the atom.134

Because the energy levels of atomic shells are unique to each element, the kinetic energy of the resulting Auger electron serves as a "fingerprint" for identification. Unlike X-ray photoelectron spectroscopy (XPS), the kinetic energy of an Auger electron is independent of the energy of the incident beam. Due to the ability to focus electron beams into small diameters, AES is highly effective for surface mapping and depth profiling, though it is generally restricted to conductors and semiconductors to avoid charge buildup.14