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 ⁴He. Dramatic differences in the onset of cluster formation between the two isotopes may be related to the stability of ⁴He dimers.
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Stephens et al. (1983) studied this question.