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How do modern spectroscopic methods identify elements on a surface?
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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.
Modern spectroscopic methods identify elements on a surface by measuring the discrete energies of electrons emitted from atoms when they are ionized by photons or electrons.12
Key Spectroscopic Techniques
- X-ray Photoelectron Spectroscopy (XPS): This method uses incident photons to eject electrons from an atom. Because different elements possess characteristic electron-binding energies, the kinetic energy of these emitted electrons allows for precise elemental identification. XPS is highly effective for qualitative analysis and can detect all elements except hydrogen and helium. It also provides information on the chemical state of elements through "chemical shifts," where the local environment of an atom slightly alters its binding energy.12
- Auger Electron Spectroscopy (AES): This technique relies on the Auger process, where an excited ion relaxes by emitting a secondary electron. Unlike XPS, the kinetic energy of an Auger electron is independent of the energy of the incident ionizing radiation. It is a standard instrumental method for determining surface composition.23
- Secondary Ion Mass Spectroscopy (SIMS): Unlike electron-based methods, SIMS interprets mass spectra to identify elements and molecules. It is the only technique sensitive to all elements and is exceptionally sensitive, capable of detecting materials at concentrations as low as one part per million of a monolayer.4