New Geometric Shell Structures

Clusters often grow by adding concentric layers of atoms in such a way that the overall symmetry of the cluster is left unchanged. Icosahedral shell structure was first envisioned by Mackay1while working on the difficult geometric problem of sphere packing. Shells with icosahedral symmetry have since been identified in clusters composed of inert-gas atoms,2–4metal atoms,5,6and even complex molecules. In this report experimental evidence will be presented for shell structures based on geometries other than the Mackay icosahedra. The results which will be discussed include: (a) octahedral shells in Al and In clusters, (b) non-Mackay-like icosahedral shells of metal deposited expitaxially on a C60molecule and (c) new data on very large cubic shells of alkali halides. In all these cases, the evidence for shells consists of mass spectrometric anomalies that appear periodically when plotted on a(mass)1/3scale. Each geometry is associated with a unique periodicity.