Scattering angle ® MEIS 2D plot of a CoAg heterogeneous alloy.

Medium Energy Ion Scattering (MEIS) is particularly sensitive to defects within a crystalline structure such as stacking faults or displaced atoms. MEIS is carried out at the facility at Daresbury Laboratory

MEIS is a refinement of the more common technique of Rutherford Backscattering (RBS), but with enhanced depth and angle resolution.

In a typical MEIS experiment a collimated beam of monoenergetic (50 - 400 keV) light ions (H⁺ or He⁺) impinges onto a crystalline sample called the target along a known crystallographic direction.

The energy and angle of the backscattered ions provide a measurement of atomic mass, depth, and structure of the target.

Ions scattered from the surface of the target loose energy in an elastic ’billiard ball’ type collision with the surface atoms. Thus, the energy of the backscattered ions relates directly to the mass of the target atoms.

Ions scattered from below the surface also loose energy inelastically at a rate proportional to the ion’s path length in the target, therefore providing a measure of the depth at which the ions were scattered.

If the incident ion beam is aligned with a high symmetry axis of the crystalline target, atoms at the surface shadow deeper atoms on lattice sites from the incident ion beam. However, target atoms, which are displaced from lattice sites, are shadowed less effectively by the surface atoms. These displaced atoms scatter the incident ions more strongly which is at the origin of the high sensitivity of MEIS to crystalline defects .

The channels provide information on the crystallinity and in this case reveal a 6-fold in-plane symmetry.
As the Co concentration was increased, the channel depth decreased indicating that the Co atoms do not sit on Ag lattice sites.