These techniques involve accelerating ions and causing them to impinge on a sample. As suggested in the names for the techniques, they are quite similar, but accelerate the ions to greater or lesser degrees. LEIS is normally considered to be 1-10keV, MEIS 20-200 keV and HEIS 200-2000 keV.

The higher the energy the smaller the target atoms appear and hence more atoms are seen (i.e. low energies are best for surface specificity). The higher the energy the smaller the cross section and hence the yield. The higher the energy the simpler it is to model the process mathematically. MEIS is sufficiently low in energy to provide surface specificity but high enough to be able to model the process accurately. MEIS is also low enough to use an electrostatic detector and have good resolution (this is not possible in HEIS). A big drawback of LEIS is that it suffers from complications due to multiple scattering.

MEIS is a refinement of the more common technique of Rutherford backscattering spectrometry (RBS), but with greatly enhanced depth (energy) resolution. In an MEIS experiment a collimated beam of mono-energetic (typically 100 keV) protons impinges onto a crystalline target along a known crystallographic direction. The energy and angle of the scattered ions are analysed simultaneously and allow MEIS to measure atomic mass, depth, and surface structure from the following physical principles;

1. Mass - ions scattered from the surface of a material undergo energy loss by a "billiard ball" type collision with surface atoms. The scattered ion energy thus relates directly to the mass of the scattering atom.

2. Depth - ions scattered from below the surface lose energy inelastically at a rate related to the ion's path length in the target. This extra energy loss thus relates directly to the depth of the scattering atom. In favourable cases MEIS can achieve a depth resolution of one atomic layer.

3. Surface structure - because the ion beam is aligned with a crystallographic axis the surface atoms shadow deeper atoms from the ion beam. This alignment therefore makes the technique surface specific and, for a particular crystal, certain ingoing directions can allow the ion beam to illuminate only the top one, two, or three layers according to choice. Ions scattered from the second layer will have their outward paths blocked at certain angles by first layer atoms. The variation in scattered ion intensity with angle thus relates to the geometrical arrangement of surface atoms. A complete solution of surface structure requires a comparison between experiment and simulation for several scattering geometries. By appropriate choice of scattering geometry atomic displacements as small as 0.05 angstroms can be measured.