So far we have concentrated on the behavior of single atoms. However, when several atoms are present on a plane and they diffuse, atoms may collide with each other and form a cluster. Such events are illustrated in Figs. 7.1, 7.2, and 7.3, where coalescence of two as well as three atoms is observed directly using the field ion microscope. These clusters are of considerable interest for the roles they play in the growth and dissolution of a crystal, as well as their effects on surface chemical reactions. Of primary concern here is the ability of atom clusters to diffuse over a crystal surface, and it is this aspect of cluster properties that we will emphasize. We will look at the conditions under which a cluster moves as a whole and also when parts of a cluster start moving independently. Different mechanisms of diffusion will be discussed in some detail.

In probing the diffusion of clusters, it is worthwhile to distinguish two different types of mechanisms – movement by single atom jumps, and by concerted atom displacements. In the first category, five types of movement have so far been identified, which are: 1. diffusion by sequential atom jumps (Fig. 7.4a), 2. peripheral displacements (Fig. 7.4b), 3. by the leapfrog mechanism (Fig. 7.4c), 4. by the correlated evaporation–condensation mechanism also known as detachment–attachment (Fig. 7.4d) or terrace limited diffusion, and 5. by the evaporation–condensation mechanism (Fig. 7.4e), in which one atom leaves a cluster and then a different atom from the terrace attaches to the cluster.