Swarm study of Kr2+ ions in helium gas at very low temperature

Doubly charged helium cluster ions Kr2+-He-n (n less than or equal to 40) h ave been produced when Kr2+ ions are injected into helium gas cooled by liq uid helium. Magic numbers n = 12 and 32 have been observed in the size dist ribution of these cluster ions. These magic numbers suggest the shell struc ture in which an icosahedron consisting of twelve helium atoms performs as a core in the cluster ion. Not only the clustering, charge transfer between Kr2+ and He also takes place as a competitive reaction of the cluster form ation. Behavior of each electronic states of the incident ions in collision s with helium have been discussed on a crude potential crossing model.