Focusing of particles scattered by a surface

It has been shown by computer simulation that the coefficient of reflection of argon atoms scattered by crystalline aluminum and germanium targets at glancing angles of less than 4$^\circ$ is close to unity and the beam of scattered particles exhibits focusing (the angular distributions of particles are strongly compressed). Whereas beam focusing with respect to the azimuth is well known and has already been studied, sharp focusing in the surface-normal direction at small glancing angles has not been studied so far. This effect is confirmed by the experimental results. It is associated with multiple scattering of incident particles by the atomic chain. The simulation results allowed finding quite accurately the amplitude of thermal vibrations of surface atoms ((0.123 $\pm$ 0.007) Å for aluminum), which agrees well with the experiment.