Medium Energy Ion Scattering (MEIS) is a relatively new technique to the UK. The Surface Physics Group is at the forefront of the use of the technique in the analytical study of surfaces and near surface interfaces.
MEIS is closely related to Rutherford Backscattering Spectroscopy (RBS), High Energy Ion Scattering (HEIS) and Low Energy Ion Scattering (LEIS). Ions (normally H+ or He+ or occasionally Li+) are accelerated to have energy of around 100 keV and scattered from the surface of interest. As ions with this energy travel at a speed much quicker than crystal phonon vibrations the interaction between ion and crystal may be considered as a sequence of kinematic scattering events between the ion and the "frozen" atomic nuclei. A more complete discussion of this scattering is given here.
MEIS becomes useful as a surface science technique by the practice of carefully aligning the ion beam with the crystal. By aligning the ion beam along a major crystal direction all but the first few atomic layers are "shadowed" (see figure 1). By detecting the scattered ions it is possible to determine information about the surface. If the scattered ions are detected around another major crytallographic direction (a so called "double alignment" geometry) then scattered ions may be seen to be "blocked" in a process analogous to shadowing. A shift in the postition of an atom will therefore cause a corresponding shift in the blocking features caused by that atom (figure 2).
A typical MEIS experiment involves detecting scattered ions over a range of energy and scattering angle simultaneously to produce a two dimensional spectrum such as that shown in figure 3. As noted in the detailed discussion the signal from each element is clearly distinguishable. Structural information is normally extracted by taking the angular cross section through the data from one element. Whilst the angular positions of blocking dips then give a guide to surface structure, this may be refined by performing a series of computer simulations of ion scattering from a number of trial structures. This allows the structural parameters to be systematically optimised.
The group's MEIS work is performed at the CLRC Daresbury MEIS facility. This offers the ability to grow samples in situ and characterise them using standard techniques such as LEED and AES before taking MEIS data. Scattering curves are simulated using the VEGAS codes, available from Daresbury and originally developed by FOM Institute, Amsterdam.
Further details of MEIS may be found in Surf. Sci., 1983, 133, p. 159 and Surf. Sci. Rep., 1985, 5, p. 199
