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How do scientists define the thickness of a surface layer in surface analysis?

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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.

In surface analysis, scientists define the thickness of a surface layer operationally as the region of a solid that differs from its bulk material. Because the depth to which surface reactions extend varies between materials, the definition of the "surface" is context-dependent, relying on the specific solid being measured and the application of the analysis.12

When using spectroscopic techniques, the "sampling depth"—which must be limited to the thickness of the surface—is determined by the penetration depth of the incident beam or the escape depth of the exiting beam. This depth varies based on the type of particle (photons, electrons, or ions) and its energy.32

For example, in catalyst studies, the surface thickness of interest may be less than 1 nm (approximately five atomic layers), whereas in corrosion studies, the depth of interest may extend to 100 nm (approximately 500 atomic layers). Additionally, classical methods such as ellipsometry are specifically employed to obtain direct thickness measurements.24