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How does ion scattering spectroscopy use reflected ions to identify surface atoms?

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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.

Ion scattering spectroscopy (ISS) identifies surface atoms by directing a beam of low-energy primary ions (typically around 1 keV) at a sample surface. When these ions strike the surface, they undergo elastic scattering.12

The technique relies on the principle that the kinetic energy of a reflected primary ion changes based on the mass of the surface atom it strikes. Specifically, the ratio of the ion's energy before and after scattering is a function of the mass of the incident ion, the mass of the surface atom, and the scattering angle. Because ions that penetrate deeper into the material lose significant energy through inelastic collisions, ISS is uniquely sensitive to the top atomic layer, as reflected ions from this layer retain the energy signature of the elastic scattering process.12

While ISS provides qualitative and semiquantitative elemental analysis for elements heavier than helium, it does not provide direct chemical information. The resulting spectra consist of elastic scattering peaks that allow researchers to determine the elemental composition of the surface.1