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).
To create the helium clusters, Toennies and colleagues in Spain and the US allowed fluid helium to expand at cryogenic temperatures and form a collimated beam. The researchers then directed this beam through a transmission grating that had a period of 100nm, which diffracted the clusters into a mass spectrometer. Since the de Broglie wavelength of a cluster is inversely proportional to its mass, clusters of different sizes were diffracted at different angles. By plotting the signal from the mass spectrometer as a function of the diffraction angle, the team was therefore able to plot the size distributions of the clusters with excellent resolution.
Contrary to what is expected, certain cluster sizes appear to be favoured more than others. Toennies and co-workers carried out several tests to check that the peaks in the size distribution were independent of conditions in the source region and in the detector. They also varied the angle of the diffraction grating relative to the incident beam of clusters, and concluded that the peaks definitely indicated magic numbers corresponding to cluster sizes of 10/11, 14, 22, 26/27 and 44 atoms.
In the July issue of Physics World Peter McClintock of the Physics Department at Lancaster University in the UK describes this work in more detail.
