Formation of Helium Cluster Ions N₂He_n⁺, COHe_n⁺, and O₂He_n⁺ (n ≤ 13 ) in Cooled Helium Gas

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Helium cluster ions have been observed when diatomic molecular ions, N₂⁺, CO⁺, and O₂⁺, are injected into a drift tube filled with helium gas cooled by liquid helium. The structures of cluster ions and three-body association rates have been discussed. Each size distribution of cluster ions XHe_n⁺ (X = N₂, CO, and O₂, n ≤ 13) in mass spectra strongly depends on the electric field strength in the drift tube. A common magic number n = 12 is observed for these three clusters, which corresponds to the icosahedral structure. Another magic number n = 2 is observed only for O₂He_n⁺, which is similar to O₂Ar₂⁺ reported as a very stable cluster ion. For each injected ion with helium atoms, the rate coefficients at 4.4 K are estimated roughly by the extrapolation method.