Ion beams are important new probes for characterizing the chemistry and structure of a wide variety of materials. When beams of particles with energies of ∼ 1000 electron volts are used, as in secondary ion mass spectrometry, it is possible to detect ions ejected from the top layer of the material with sensitivities well below the picogram level. A number of theoretical developments now permit analysis of the geometry of adsorbed atoms and molecules on surfaces from the angular distributions of the ejected particles. Much surface chemical information can also be deduced from ejected molecular cluster ions. In addition, the observation of clusters with weights up to nearly 20,000 atomic mass units promises to expand applications of mass spectrometry to the analysis of biomolecules and the sequencing of proteins.
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