Effect of isotopic content on the rate constants for the dissociative electron–ion recombination of N2H+

The rate constants, αe, for dissociative electron–ion recombination of various isotopic combinations of N2H+ have been determined at 300 and 500K in a variable temperature flowing afterglow using a Langmuir probe to determine the electron density. The values of αe at 300K are 2.77 (14N2H+), 2.12 (14N2D+), 2.31 (15N2H+) and 1.98 (15N2D+)×10−7cm3s−1. The equivalent values at 500K are 2.84, 2.33, 2.93, and 2.33 respectively. This has shown that the greatest change occurs between H and D substitution αe(14/15N2D+)/αe(14/15N2H+)∼0.765/0.857 at 300K and 0.820/0.795 at 500K respectively. Values at 500K are consistently larger than at 300K. At both temperatures, the rate constants with H substitution are larger than with D substitution for both 14N and 15N. Values with 14N substitution are larger than with 15N for both H and D. At 500K, the values are independent of whether the ion contains 14N or 15N. Gas-phase 15N fractionation enhancement is predicted in many regions of the interstellar medium including cold and pre-stellar cores. The effect of this fractionation on the recombination process is investigated. The relevance to storage ring measurements of recombination rate constants is considered and applications to the chemistry of the interstellar medium are discussed.