Collision processes of singly and multiply charged ions in cold helium gas

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We are aiming at studying electronic energy transfer of singly and multiply charged ions in collision with cold helium gas at 3-300K. Although pair-potentials between helium atom and other atomic species are relatively well known among other interatomic potentials, experimental studies of collisions between cold helium atoms and other atoms are scarce. Moreover, only a few collision cross sections are measured for the combination of helium atom and atomic ions even at room temperature. This is presumably because of the difficulty of producing atomic ions in the environment of cold helium gas. In order to solve the problem, we applied laser ablation technique to a solid sample in cold helium gas to produce atomic ions and detected them with laser-spectroscopic method. The results will leads to the information on interatomic potentials between ions and atoms.Produced ions are excited to the electronically excited state with a tunable dye laser, then both laser-induced fluorescence (LIF) from ions in the excited state and sensitized fluorescence (SF) from ions in the state to which ions are transferred by collision with helium atoms are observed simultaneously. This procedure makes it possible to determine collision cross sections of ions and cold helium atoms. In particular, alkali-earth ions such as Ba^+ reveal fascinating character of forming molecules called exciplex (e.g. Ba^<+*>-He) in high density cold helium gas due to the three-body collision of excited ions and two helium atoms. Another two-body collision process of such exciplex and helium atoms results in dissociation of Ba^<+*>-He into Ba^+ ion and helium atom. We have clarified a peculiar temperature dependence of cross sections caused by two- and three-body collisions at temperature 3-30 K for the first time.