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This chapter presents a brief introduction to ion scattering. The wavelength for ions with an energy of at least 500 eV is so low, that both imaging and diffraction is beyond the experimental possibilities. The interaction of the incoming ion with a surface atom is described by the collision of two (practically free) atoms. By adding up all possible paths it is seen, that an area behind the surface atom cannot be entered, this area is called a shadow cone. For low ion energies the angle of incidence is varied. The scattering rate yields the number of surface atoms at the edge of the shadow cone at the surface for all angles. Deviations from the distribution as expected for a perfect surface provides structural information also on defects like thermal vibrations or melting (Ion Scattering Spectroscopy ISS, Impact Collision Ion Scattering Spectroscopy ICISS). For high ion energies, the shadow cone is very narrow, so that for special crystallographic directions all atoms on lattice sites in lower layers are completely shadowed by a perfect top layer (channeling direction). Fairly high defect concentrations are needed, due to the depth resolution also thicker and hidden surfaces may be studied (e.g. surface melting or interfaces). |
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