Clusters of He atoms formed in a supersonic free jet expansion have been studied by electron-bombardment ionization and mass spectrometry. Distinct magic-number enhancements of ion intensity are observed for clusters of 7, 10, 14, and 30 atoms for both helium isotopes, and for 23 atoms of ${}_{}{}^4{}_{}{}^{}\mathrm{He}$. Dramatic differences in the onset of cluster formation between the two isotopes may be related to the stability of ${}_{}{}^4{}_{}{}^{}\mathrm{He}$ dimers.

• Received 15 June 1983

DOI:https://doi.org/10.1103/PhysRevLett.51.1538