DIMER EMISSION IN ALLOY SPUTTERING AND THE CONCEPT OF THE CLUSTERING PROBABILITY

Using molecular-dynamics simulation, we study dimer emission from 1 keV ion bombarded Cu, and from a number of Cu-based model alloys. To analyze the dimer yields, we introduce the concept of a clustering probability p, which we define as the ratio of the dimer yield to the number of potential dimers, i.e., the number of sputtered atom pairs. For alloy bombardment, we find that p is independent of the surface binding energy of the dimer, which we define as the sum of the surface binding energies of the component atoms; p increases monotonically with the dissociation energy of the dimer. It is rather independent of the dimer mass, if an isotopic mixture of light and heavy Cu is taken as target. These results show that the clustering probability is a good concept to rationalize alloy dimer yields. We investigate this concept in further detail for dimer emission from pure Cu. It is shown how p is related to the sputter statistics. We discuss p in terms of the spatial correlation of the two atoms forming the cluster on the target surface: most sputtered dimers consist of atoms which have been neighbours on the target surface; if a whole set of nearest-neighbour atoms is emitted, the probability of dimer formation among these atoms is considerably increased.