On glancing scattering of light ions from tungsten single crystal surfaces

A computer calculation of light ion (200–800 keV protons and helium) scattering from a tungsten single crystal model with a (110)-type surface is presented. An axis (100) is considered. Thermal vibrations in 3 dimensions have been incorporated. The number, i.e. fraction, and spatial distribution of reflected ions for some angular scans are presented in an appendix. The importance of trajectory histories is demonstrated. 3 regimes of scattering behaviour as a function of the incidence angle ψ in to the string in the surface are found:

(i) for very small ψ_in surface-semi-proper-channelling of all ions;

(ii) for intermediate ψ_in mainly planar channelling and dechannelling resulting in a decreasing reflective index for increasing ψ _in;

(iii) axial channelling and transverse plane focussing are mainly of importance leading to a small reflective index, and surface semi-channelling like for (ii) occurs for very few ions.

The importance of the scattering potential is demonstrated and an expression is discussed.