Angular straggling in grazing scattering of fast He-atoms from an Al(111)-surface

We have studied the scattering of 2.3-8.0 keV neutral He-atoms from Al(111) under glancing angles of incidence (0.4 degrees-2 degrees to the surface plane). The polar angular distributions of scattered particles, measured in the plane defined by the incident beam direction and the surface normal, show a peak for specular reflection and exhibit small widths (about 0.1 degrees FWHM for 0.4 degrees incidence angle). Computer simulations reproduce the experiments in detail, showing that the angular straggling is caused by multiple scattering at 1) thermally displaced surface atoms, and 2) conduction electrons, where the latter mechanism seems to be dominant for small incidence angles. At high beam energies the observed angular straggling is larger than the simulated one, which may be due to fluctuations in the charge state of the projectiles and electronic excitations of the target.