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Physical Review A

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

Yoshimitsu Fukuyama, Yoshiki Moriwaki, Yukari Matsuo

Published Mar 30, 2007• 7 citations
Publisher

Abstract

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.

DOI
10.1103/physreva.75.032725

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Laser induced fluorescence spectra of Ag atoms and their time profiles have been studied in cold He gas at temperatures 4.3--25 K and pressure $\ensuremath{\lesssim}1\mathrm{atm}$. The emission spectra following the excitation of the atomic D2D2D2 line consist of three narrow lines and three broadbands, which are assigned to transitions of Ag atoms and AgHe and AgHe2{\mathrm{AgHe}}_{2}AgHe2​ exciplexes. Surprisingly long ( $\ensuremath{\gtrsim}300\mathrm{ns}$) effective emission lifetimes are observed at T>TcT>{T}_{c}T>Tc​ (5.2 K). The long lifetimes are the result of population trapping in the 2D5/2{}^{2}{D}_{5/2}2D5/2​ metastable state. At $T\ensuremath{\lesssim}{T}_{c}$, the effective lifetimes decrease to $\ensuremath{\lesssim}20\mathrm{ns}$ due to the condensation of He gas on the Ag atom in its 2D5/2{}^{2}{D}_{5/2}2D5/2​ state.