Molecular fragmentation by recombination with cold electrons studied with a mass sensitive imaging detector

The recombination of a molecular cation with a low-energy electron, followed by fragmentation, is a fundamental reaction process in cold and dilute plasmas. For polyatomic ions, it can yield molecular fragments in ro-vibrationally excited states. The discrimination between decay channels with chemically different fragments and the measurement of their excitation energies pose an experimental challenge. This work discusses a new experimental scheme based on fast beam fragment imaging in a storage ring with a silicon strip detector. The working principle of the detector and analysis procedures are discussed, and the performance is demonstrated in an experiment on the dissociative recombination of CHD+. Moreover, the new arrangement is used to study the dissociative recombination of DCND+. Hydrogen cyanide (HCN) and its energetically higher lying isomer hydrogen isocyanide (HNC) play an important role in the chemistry of dense interstellar clouds. It has been proposed that both isomers are formed with the same efficiency in dissociative recombination of HCNH+. For the first direct investigation of the products of this reaction channel, the new detector is used to analyse the dissociative recombination of DCND+. The results show that DCN/DNC is mostly produced in vibrationally excited states, well above the isomerization barrier. The implications of this fi nding on branching ratios and the dissociation mechanism are discussed