Introduction

The present chapter is devoted to the comparatively new and rapidly developing subject of clusters, a field intermediate between atomic physics, chemistry and solid state physics, in which concepts borrowed from nuclear physics have also proved very important. Although the field is new, it has expanded very rapidly, and there are many different aspects beyond the scope of the present book. We therefore confine our attention to: (i) a general introduction and (ii) some aspects of cluster physics which are specifically connected with material already presented in the previous chapters.

A cluster is an assembly of identical objects whose total number can be chosen at will. An atomic cluster is therefore an assembly of atoms in which the total number is adjustable. Just as, in solid state physics, one distinguishes between cases in which the valence electrons become mobile and those in which they remain localised on individual atomic sites, so one finds different kinds of clusters, depending on the degree of localisation of the valence electrons. Broadly speaking, these differences are dictated by the periodic table: at one extreme, one has the rare-gas clusters, in which electrons remain localised, while at the other, one finds the alkali clusters, which are metallic in the sense that the valence electrons can move throughout the cluster.

The subject of atomic clusters arose only recently because it was not appreciated in earlier times that identical atoms could hang together in this way.