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The total attenuation cross section sigma att and the single-electron capture cross section sigma 21 in collisions of 3He2+ with H2 have been measured using a tandem mass spectrometer whose collision cell consists of an octopole ion beam guide. A possible scattering loss which might contribute to sigma att is examined, and a large part of the difference sigma att- sigma 21 is attributed to the double-electron capture process. As its magnitude is as large as 10-15 cm2 at the lowest energy studied. The importance of the process is emphasized in connection with the production of fast protons through Coulomb explosion of H2+2.

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Abstract. Rare gas ions Ne ÷, Ar ÷ and Kr ÷ are injected into a drift tube which is filled with helium gas and cooled by liquid helium. Helium cluster ions RgHe~ + (Rg = Ne, Ar and Kr, x < 14) are observed as products. Information regarding the stability of RgHef is obtained from drift field dependence of the size distribution of the clusters, and magic numbers are determined. The magic numbers are x = 11 and 13 for NeHe~ + and x= 12 for ArHe~ + and KrHe~ + . NeHe~ + , Ar + and Kr + are proposed as the core ions for NeHe~3, ArHe~2 and KrHei~2, respectively.