The energy-resolved rate coefficient for the dissociative recombination (DR) of $\mathrm{H}_3{}^{+}$ with slow electrons has been measured by the storage-ring method using an ion beam produced from a radiofrequency multipole ion trap, employing buffer-gas cooling at 13 K. The electron energy spread of the merged-beams measurement is reduced to $500\mu \mathrm{eV}$ by using a cryogenic GaAs photocathode. This and a previous cold-$\mathrm{H}_3{}^{+}$ measurement jointly confirm the capability of ion storage rings, with suitable ion sources, to store and investigate $\mathrm{H}_3{}^{+}$ in the two lowest, $(J,G)=(1,1)$ and $(1,0)$ rotational states prevailing also in cold interstellar matter. The use of para-${\mathrm{H}}_2$ in the ion source, expected to enhance para-$\mathrm{H}_3{}^{+}$ in the stored ion beam, is found to increase the DR rate coefficient at meV electron energies.

• Received 17 September 2005

DOI:https://doi.org/10.1103/PhysRevLett.95.263201