Surface Diffusion of Compact and Other Clusters:Irxon Ir(111)

Quantitative measurements have been made of the diffusivity $D$ of larger Ir clusters on Ir(111) to explore the mechanisms of motion. For noncompact ${\mathrm{Ir}}_{18}$, migration occurs by changes in overall shape as atoms diffuse along cluster edges; in an Arrhenius plot, the prefactor $D_0$ is $\sim {10}^{-3\mathrm{}}{\mathrm{cm}}^2/\mathrm{s}$, comparable to that for single atom motion. For ${\mathrm{Ir}}_{19}$ and ${\mathrm{Ir}}_7$, clusters which exist as compact closed shells, diffusion by changes in overall shape is shown to be unlikely. Migration occurs by an entirely different mechanism, with prefactors $D_0$ being 3 or more orders of magnitude higher than for ${\mathrm{Ir}}_{18}$.