Formation and dissociation of Ba+* -He exciplexes at 3-25 K

We report the formation rate constants and the dissociation cross sections of Ba+* -He exciplexes in an environment of cold gaseous helium at temperatures of 3-25 K. We have observed laser-induced fluorescence spectra of Ba+ and Ba+* -He exciplexes by means of D2 excitation of Ba+ ions. Using the rate equation analyses of fluorescence intensities of Ba+* -He and Ba+, we have determined the rate constant of exciplex formation (Ba+: 6p P 32 2 +2He→ Ba+* -He: 6p Π 32 2 +He) attributable to three-body collision at 5, 10, 15, and 20 K. The formation rate constant was found to have irregular temperature dependence, with a local maximum kf = (6.0±0.8) × 10-44 m6 s-1 at 15 K. Results of this study also indicate peculiar temperature dependence of the dissociation cross section (Ba+* -He: 6p Π 32 2 → Ba+: 6p P 12 2 +He). It has a local maximum σdis = (3.1±0.4) × 10-21 m2 at 3.6 K. In cold He gas at 25 K or lower, a new channel of population transfer from the 6p P 32 2 state to the 6p P 12 2 state of Ba+ is opened because both exciplex formation and dissociation start to occur frequently.