Angular dependence of clusters sputtered from a tungsten single crystal surface

The anisotropy of positively charged clusters sputtered with 150 keV Ar+ ions from a tungsten single crystal has been investigated with a mass and energy spectrometer. Not only the monomer W+ but also the clusters W+ 2, W+ 3 and W+ 4 (so far investigated) show maxima of emission along close packed directions. The maximum cluster yield along a <111> direction normalized to the yield along a random direction increases with increasing cluster size and cluster energy, although the absolute cluster yield decreases. An interpretation of the experimental results on the basis of a simple statistical model of cluster formation kinetics is presented which explains the yield profiles of the anisotropic contribution fairly well.