Formation of helium cluster ions N2Hen+ and COHen+, and O2Hen+ (n <= 13) in cooled helium gas

Helium cluster ions have been observed when diatomic molecular ions, N-2(+) , CO+, and O-2(+), are injected into a drift tube filled with helium gas co oled by liquid helium. The structures of cluster ions and three-body associ ation rates hale been discussed. Each size distribution of cluster ions XHe n+ (X = N-2, CO, and O-2, n less than or equal to 13) ill mass spectra stro ngly depends on the electric field strength in the drift tube. A common mag ic number n = 12 is observed for these three clusters, which corresponds to the icosahedral structure. Another magic number n = 2 is observed only rep orted as a very stable cluster ion. For each injected for O2Hn+, which is s imilar to O2Ar2+ ion with helium atoms, the rate coefficients at 4.4K are e stimated roughly by the extrapolation method.