Abstract
This paper presents the first dissociative recombination (DR) measurement of electrons with rotationally and vibrationally cold $\mathrm{H}_{3}^{+}$ ions. A dc discharge pinhole supersonic jet source was developed and characterized using infrared cavity ringdown spectroscopy before installation on the CRYRING ion storage ring for the DR measurements. Rotational state distributions $({T}_{\mathrm{rot}}\ensuremath{\sim}30\phantom{\rule{0.3em}{0ex}}\mathrm{K})$ produced using the source were comparable to those in the diffuse interstellar medium. Our measurement of the electron energy dependence of the DR cross section showed resonances not clearly seen in experiments using rotationally hot ions, and allowed calculation of the thermal DR rate coefficient for ions at interstellar temperatures, ${\ensuremath{\alpha}}_{\mathrm{DR}}(23\phantom{\rule{0.3em}{0ex}}\mathrm{K})=2.6\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}7}\phantom{\rule{0.3em}{0ex}}{\mathrm{cm}}^{3}\phantom{\rule{0.3em}{0ex}}{\mathrm{s}}^{\ensuremath{-}1}$. This value is in general agreement with recent theoretical predictions by Kokoouline and Greene [Phys. Rev. A 68, 012703 (2003)]. The branching fractions of the two breakup channels, $\mathrm{H}+\mathrm{H}+\mathrm{H}$ and $\mathrm{H}+{\mathrm{H}}_{2}$, have also been measured for rotationally and vibrationally cold $\mathrm{H}_{3}^{+}$.
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