Formation of small metal clusters by ion bombardment of single crystal surfaces

The mechanism for the formation of small metal clusters ejected from an ion bombarded metal surface is examined in detail. The analysis is performed by classical trajectory methods which determine the positions and momenta of all particles in a model microcrystallite as a function of time. The calculation utilizes pair potentials for Cu derived from elastic constants of the solid and is performed for 600 eV Ar⁺ ion at normal incidence to the crystal. The results show that cluster species do not leave the surface as intact parts of the solid but form in a region above the surface. A trajectory for Cu ³ formation is traced in detail showing a typical mechanism which is valid for Cu_n formation where n ≤ 7.