How can a complete shell of surrounding particles cause molecular fragments to recombine?

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cluster Figure 1: The four stable geometric structures of the seven-atom cluster of argon, in order of increasing energy: (A) A pentagonal bipyramid. (B) A regular octahedron with one face capped by the seventh atom. (C) A regular tetrahedron with three of its faces capped by other atoms. (D) A trigonal bipyramid with two of its faces capped by other atoms; although this has the highest energy of the four structures, it is very close in energy to the tricapped tetrahedron.

When a molecule within a cluster is dissociated—for instance, by light of a sufficiently short wavelength—the surrounding atoms or molecules act as a cage. If this surrounding layer is incomplete, the resulting fragments often escape the cluster entirely.

However, if the cluster contains enough nonreacting atoms or molecules to form at least one complete shell around the dissociating molecule, the cage effectively traps the fragments. By holding these fragments in close proximity, the shell prevents them from flying apart, allowing them to remain together until they eventually recombine.1