Over-cosine angular distributions of sputtered atoms at normal incidence

The angular distribution of sputtered atoms for normal incidence ions has been investigated theoretically and by computer simulation. For low energy ions the angular distribution is under-cosine, while for relatively high energy ions we obtain an over-cosine angular distribution for the sputtered atoms. It is found that the outward-peakness of the angular distribution for relatively high energy ions is due to the geometrical asymmetry near the surface. Using the Monte Carlo simulation code ACAT, which is based on the binary collision approximation, the angular distributions of sputtered atoms are calculated for various incident energies of Ar ions incident normally on a Fe target. It is found that one needs to take into account the surface roughness in order to obtain good agreement with experiment. The surface roughness is believed to reduce the degree of the over-cosine distribution because a rough surface has a larger effective surface area as compared with an unirradiated surface. (orig.)