Diffusion of Large Surface Clusters: Direct Observations on Ir(111)

To probe if cluster diffusion is possible without evaporation and condensation of atoms, observations with atomic resolution have been made on clusters on Ir(111) containing from 18 to 39 Ir atoms. For clusters which cannot assume a hexagonal structure, diffusion at $T\ge 550\mathrm{K}$ takes place by atoms moving along the edges, changing the cluster shape. For I${\mathrm{r}}_{19}$, with a compact hexagonal arrangement of atoms, no shape changes are observed. Nevertheless, diffusion occurs without loss of atoms, with the cluster frequently moving over distances larger than a nearest-neighbor spacing.