Collective Resonances in Metallic Clusters

Abstract

A cluster is a group of atoms held together by forces which do not saturate, by which one means that one can always add one more atom to a cluster without altering its basic properties. This feature is called stackability [1]. The implication of stackability is that valence is not truly relevant for clusters and this is one of the basic differences between atomic clusters and molecules. Molecular bonds are often directional, and molecular valence considerably restricts the combinations of atoms which can be assembled. Thus, it is not usually possible for the atoms in a molecule to be completely stackable, although some molecules come much closer than others to satisfying this requirement. In what follows, I will take the view that stackability is the defining property of clusters, and provides an angle from which most of their physics should be viewed.