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Magical helium clusters

Jul 10, 2004

Physicists are well acquainted with the concept of "magic numbers". In nuclear physics, for example, a nucleus is more stable if it contains a magic number (e.g. 2, 8, 20, 28,...) of protons or neutrons. Similar patterns are seen in small atomic systems, such as the closed shells of valence electrons in metal clusters. Theory predicts, however, that such numbers -- which are related to an enhanced stability of the ground state -- should not occur for clusters of helium-4 atoms. Rather, the ground-state properties of the cluster should change smoothly as the number of atoms it contains increases. Now, astonishingly, Peter Toennies of the Max Planck Institute for Flow Research in Göttingen, Germany, and co-workers have observed magic numbers in helium-4 clusters (R Brühl et al. 2004 Phys. Rev. Lett. 92 185301).

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