SURFACE-DIFFUSION OF COMPACT AND OTHER CLUSTERS - IR-X ON IR(111)

Quantitative measurements have been made of the diffusivity D of large r Ir clusters on Ir(lll) to explore the mechanisms of motion. For nonc ompact Iris, migration occurs by changes in overall shape as atoms dif fuse along cluster edges; in an Arrhenius plot, the prefactor D-0 is s imilar to 10(-3) cm(2)/s, comparable to that for single atom motion. F or Ir-19 and Ir-7, clusters which exist as compact closed shells, diff usion by changes in overall shape is shown to be unlikely. Migration o ccurs by an entirely different mechanism, with prefactors D-0 being 3 or more orders of magnitude higher than for Ir-18. [S0031-9007(98)0775 9-X].