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This chapter talks about medium and high energy ion scattering (MEIS and HEIS). MEIS and HEIS spectrometry and LEED intensity-voltage (IV) analysis are the two main experimental techniques for surface relaxation determination. LEED reveals long range order and MEIS and HEIS reveal short range order in a surface. Since the penetration depth of the probing ions (usually 0.05 to 3 MeV H+ or He+) is between hundreds of nanometers and several micrometers, MEIS and HEIS as such are not surface analysis techniques. The distinction between MEIS and HEIS is mainly instrumental; the underlying physics is the same and, therefore, treated here together. Remarkable for MEIS/HEIS is the simplicity of the underlying physics. For medium and high energies, neutralization is not excessively high. Moreover, it is independent of the particle’s trajectory. Of course, if a solid state detector is used, both backscattered ions and neutrals are detected without any discrimination. Furthermore, the collisions of the ions in the crystal can be fully regarded as binar; collisions. And, since scattering cross sections are relatively small, almost all detected ions are backscattered in one single hard collision: double and multiple scatterings are rare. The required ion accelerator and beam steering optics are in contrast more complicated and expensive than for LEIS. |
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