Absolute doubly differential angular sputtering yields for 20 keV Kr+ impacting a loose Cu powder

We have measured the absolute doubly differential angular sputtering yield for 20 keV Kr+ impacting a loose Cu powder at an incidence angle of θi = 45°. We find that ion sputtering from a loose Cu powder differs dramatically from a flat Cu slab under the same irradiation conditions, being most pronounced for the angular distribution. The ejecta lobe for the powder is directed primarily backward, relative to the ion velocity vector. This contrasts with the slab, where the lobe is peaked nearly normal to the surface. We attribute the backward-directed ejecta lobe to two effects. First, ions can penetrate through the interconnected voids of the powder and cause sputtering from underlying grains. Atoms sputtered from these underlying grains are most likely to escape the powder if they are ejected roughly toward the origin of the incident ions, while ejecta in other directions are more likely to be shadowed by neighboring grains. Second, for θi = 45°, the grain surfaces exposed to the ions are primarily oriented in the backward direction, leading to shadowing of forward-directed ejecta. We find qualitatively good agreement between our experimental measurements and the results of our multiscale Monte Carlo simulation of sputtering from granular targets. We attribute the remaining discrepancies to ion-induced surface modifications not included in the simulations. Our findings indicate that ion sputtering from loose powders is distinctly different from that of flat surfaces, rough surfaces, and pressed powders due to the lack of interconnected voids in these latter three targets.