Molecular ion irradiations of molybdenum

A series of TEM experiments was carried out to systematically investigate the formation of vacancy dislocation loops in displacement cascades in molybdenum. Single-crystal foils of high-purity molybdenum were irradiated with Sb+ single ions and Sb-2(+) and Sb-3(+) molecular ions to low doses (<= 10(16) ions m(-2)). Three different ion energies were employed (60, l20 and 180 keV) in order to systematically vary the total cascade energy and the energy per atom in the molecular ion. Dislocation loop sizes and defect yields were found to be larger for molecular ions than for single ions of the same energy. In (011) foils, most loops had Burgers vectors b = a/2 < 111 > lying in the plane of the foil. However, in molecular ion irradiations, a small fraction of loops with b = a < 100 > was also found. This fraction was higher for Sb-3(+) than for Sb-2(+) ions and also increased with ion energy. In (001) foils, defect yields were much smaller because of the loss of glissile a/2 < 111 > loops to the surface, but a < 100 > loops were still present in molecular ion irradiations. The habit planes of both a/2 < 111 > and a < 100 > loops were consistent with nucleation on {110} planes by the Eyre-Bullough mechanism. These results are compared with recent molecular dynamics simulations of the effect of the mass of primary knock-on atoms on displacement cascades in iron.