THE COLOR OF METAL-CLUSTERS AND OF ATOMIC-NUCLEI

The term 'clusters' is usually used to describe aggregates of atoms that are too large to be referred to as molecules and too small to resemble small pieces of crystal. These aggregates generally do nor have the same structure or atomic arrangement as a bulk solid and can change structure with the addition of just one or a few atoms. As the number of atoms increases, eventually a crystal-like structure is established, and the addition of new atoms does not change the bulk structure. A motivation for studying clusters is the desire to understand how physical properties evolve in the transition from atom to molecule to cluster to small particle to solid. At present one has a good understanding of the endpoints of such an evolution, but has still a poor knowledge of the intermediate situations. In particular, while one understands reasonably well why a sodium atom is yellow and why this colour essentially is absent in the electromagnetic spectrum of bulk sodium, we still do not have an accurate theory which explains the spectrum of colours displayed by small sodium clusters. Much progress towards this goal can be achieved by making use of well-established results in nuclear physics. It turns out that there is a close analogy between metal clusters and atomic nuclei, and knowledge acquired during the past 40 years in the study of the response of nuclei to photons helps with the understanding of the absorption of light by microcrystals. The main result of developing such an analogy seems to be that the properties of finite, many-particle systems do not depend so much on the nature of the particles forming the system or the forces acting among them, but on the fact that they move in a confined volume and that they are many of them.