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 particle s, measured in the plane defined by the incident beam direction and th e surface normal, show a peak for specular reflection and exhibit smal l widths (about 0.1 degrees FWHM for 0.4 degrees incidence angle). Com puter simulations reproduce the experiments in detail, showing that th e 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 be am energies the observed angular straggling is larger than the simulat ed one, which may be due to fluctuations in the charge state of the pr ojectiles and electronic excitations of the target.