Dissociative Recombination of N2H+: Evidence for Fracture of the N-N Bond

Branching ratios and absolute cross sections have been measured for the dissociative recombination of N2H+ using the CRYRING ion storage ring. It has been found that the channel N2H+ + e- → N2 + H accounts for only 36% of the total reaction and that the branching into the other exoergic pathway, N2H+ + e- → NH + N, consequently amounts to 64%. The cross section of the reaction could be fitted by the expression σ = (2.4 ± 0.4) × 10-16E-1.04±0.02 cm2, which leads to a thermal reaction rate of k(T) = (1.0 ± 0.2) × 10-7(T/300)-0.51±0.02 cm3 s-1, in favorable agreement with previous flowing afterglow Langmuir probe measurements at room temperature, although our temperature dependence is very different. The implications of these measurements for the chemistry of interstellar clouds are discussed. A standard model calculation for a dark cloud predicts a slight increase of N2H+ in the dark clouds but a five- to sevenfold increase of the NH concentration as steady state is reached.