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How do beam-in and beam-out spectroscopic techniques study a solid 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.

Spectroscopic techniques for surface analysis utilize a "beam-in, beam-out" mechanism to probe the composition of a solid surface. In this process, a primary beam—consisting of photons, electrons, or ions—is directed at the material.1

As the incident beam penetrates the solid, it interacts with the material, causing a second beam to be emitted from the surface. This exiting beam is captured and analyzed by a spectrometer, which extracts information about the chemical composition of the region where the interaction occurred.1

To ensure the technique is surface-specific, the penetration depth of the incident beam or the escape depth of the exiting beam must be limited to the thickness of the surface layer. This "sampling depth" varies based on the type and energy of the particles used. By adjusting these parameters, researchers can employ various techniques, such as:1

X-ray photoelectron spectroscopy (XPS): Uses photons in and electrons out.1
Auger electron spectroscopy (AES): Uses electrons in and electrons out.1
Secondary ion mass spectroscopy (SIMS) and ion scattering spectroscopy (ISS): Use ions in and ions out.1