Molecular dynamics simulation of preferential sputtering from isotopic mixtures

The nonstoichiometric sputtering of different isotopes from elemental targets containing two and three isotopes is simulated with molecular dynamics. Cu targets consisting of isotopes with artificially high mass differences ranging from 10% to 30% are bombarded with normally incident 5 keV Ar+ ions. Both liquid and single crystal targets are studied. Both backward and forward sputtering spectra are examined. Overall enrichments of the lighter isotopes in the sputtering material are observed in all cases with a consistent lower enrichment in the material sputtered forward for the liquid targets. Simulation results show a linear dependence of the isotopic fractionation (fractional change of the isotopic ratio in the sputtered atoms from the stoichiometric value) on the mass difference in the three-isotope system within statistical uncertainties. A clear dependence of the isotopic fractionation on the angle of emission with a larger enhancement of the lighter isotope in materials ejected toward the surface normal is observed for the crystal targets but not for liquid. For the liquid targets, a larger isotopic fractionation in the backward sputtering yield is seen. An ion mass dependence is also noted with a higher fractionation effect for the lighter ion. (orig.)