Surface diffusion of atom clusters

Quantitative measurements have been made of the motion of tungsten doublets and triplets on the (211) plane of tungsten. It is shown that these clusters diffuse by the movement of one atom at a time. The activation energy (35 kJ mol-1) and prefactor (10-11 cm 2 s-1) of the diffusion coefficient are the same for both doublets and triplets, but differ substantially from the parameters characteristic of single-atom motion. This behaviour is interpreted in terms of strong attractive interactions between pairs of atoms.