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Formation and dissociation of Ba + * − He exciplexes at 3 – 25 <…

2007; American Physical Society; Volume: 75; Issue: 3 Linguagem: Inglês

10.1103/physreva.75.032725

1538-4446

Yoshimitsu Fukuyama, Yoshiki Moriwaki, Yukari Matsuo,

Atomic and Subatomic Physics Research

We report the formation rate constants and the dissociation cross sections of ${\mathrm{Ba}}^{+*}\text{\ensuremath{-}}\mathrm{He}$ exciplexes in an environment of cold gaseous helium at temperatures of $3--25\phantom{\rule{0.3em}{0ex}}\mathrm{K}$. We have observed laser-induced fluorescence spectra of ${\mathrm{Ba}}^{+}$ and ${\mathrm{Ba}}^{+*}\text{\ensuremath{-}}\mathrm{He}$ exciplexes by means of $D2$ excitation of ${\mathrm{Ba}}^{+}$ ions. Using the rate equation analyses of fluorescence intensities of ${\mathrm{Ba}}^{+*}\text{\ensuremath{-}}\mathrm{He}$ and ${\mathrm{Ba}}^{+}$, we have determined the rate constant of exciplex formation (${\mathrm{Ba}}^{+}$: $6p$ $^{2}P_{3∕2}+2\mathrm{He}\ensuremath{\rightarrow}{\mathrm{Ba}}^{+*}\text{\ensuremath{-}}\mathrm{He}$: $6p$ $^{2}\ensuremath{\Pi}_{3∕2}+\mathrm{He}$) attributable to three-body collision at 5, 10, 15, and $20\phantom{\rule{0.3em}{0ex}}\mathrm{K}$. The formation rate constant was found to have irregular temperature dependence, with a local maximum ${k}_{f}=(6.0\ifmmode\pm\else\textpm\fi{}0.8)\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}44}\phantom{\rule{0.3em}{0ex}}{\mathrm{m}}^{6}\phantom{\rule{0.2em}{0ex}}{\mathrm{s}}^{\ensuremath{-}1}$ at $15\phantom{\rule{0.3em}{0ex}}\mathrm{K}$. Results of this study also indicate peculiar temperature dependence of the dissociation cross section (${\mathrm{Ba}}^{+*}\text{\ensuremath{-}}\mathrm{He}$: $6p$ $^{2}\ensuremath{\Pi}_{3∕2}\ensuremath{\rightarrow}{\mathrm{Ba}}^{+}$: $6p$ $^{2}P_{1∕2}+\mathrm{He}$). It has a local maximum ${\ensuremath{\sigma}}_{\mathrm{dis}}=(3.1\ifmmode\pm\else\textpm\fi{}0.4)\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}21}\phantom{\rule{0.3em}{0ex}}{\mathrm{m}}^{2}$ at $3.6\phantom{\rule{0.3em}{0ex}}\mathrm{K}$. In cold He gas at $25\phantom{\rule{0.3em}{0ex}}\mathrm{K}$ or lower, a new channel of population transfer from the $6p$ $^{2}P_{3∕2}$ state to the $6p$ $^{2}P_{1∕2}$ state of ${\mathrm{Ba}}^{+}$ is opened because both exciplex formation and dissociation start to occur frequently.