Diffusivities have for the first time been quantitatively measured for twohyphen;dimensional clusters of iridium atoms on a metal surface, Ir(111). As cluster size increases from two to seven atoms, the diffusivity generally diminishes, but tetramers are an exception. It is shown that their high mobility comes about because of weak interatomic binding, and a low activation energy for diffusion, rather than from a change in the mechanism of motion, which involves displacements of individual adatoms independent of cluster size.
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