Formation of Helium Cluster Ions⁴Hen⁺(n≤15) in a Liquid Helium Cooled Drift Tube

Helium cluster ions 4 He n + (2≤ n ≤15) have been formed in a drift tube. Information about binding energy, Δ E n = E n - E n -1 , has been obtained. The binding energy drastically decreases at n =4, then gradually decreases with increasing n . Steplike changes have also been observed between n =10 and 11 and n =14 and 15, which indicates that the cluster sizes n =10 and 14 are magic numbers. Geometrical structures of He 10 + and He 14 + are discussed.