Upper limit of a tunneling reaction rate for D−+H2→HD+H−

The exothermic proton transfer reaction ${\mathrm{D}}^{\ensuremath{-}}+{\mathrm{H}}_{2}\phantom{\rule{4pt}{0ex}}\ensuremath{\rightarrow}\text{HD}+{\mathrm{H}}^{\ensuremath{-}}$ is known to proceed over a barrier of about 0.33 eV. Here we investigate whether this reaction may occur at low temperatures via tunneling through this barrier. The experiments were carried out in a cryogenic 22-pole ion trap, which provides a high sensitivity for slow ion-molecule reactions. Our experiments show no sign of the tunneling reaction with an upper limit to the rate coefficient of $2.6\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}18}$ ${\mathrm{cm}}^{3}/\mathrm{s}$ obtained from the decrease of the ${\mathrm{D}}^{\ensuremath{-}}$ signal and $9\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}19}$ ${\mathrm{cm}}^{3}/\mathrm{s}$ from the absence of an increase of ${\mathrm{H}}^{\ensuremath{-}}$. Background impurities were identified to be the main limitation of the sensitivity.