Effect of isotopic content on the rate constants for the dissociative electron–ion recombination of N 2H +

Abstract

The rate constants, α e, for dissociative electron–ion recombination of various isotopic combinations of N 2H + have been determined at 300 and 500 K in a variable temperature flowing afterglow using a Langmuir probe to determine the electron density. The values of α e at 300 K are 2.77 ( 14N 2H +), 2.12 ( 14N 2D +), 2.31 ( 15N 2H +) and 1.98 ( 15N 2D +) × 10 −7 cm 3 s −1. The equivalent values at 500 K are 2.84, 2.33, 2.93, and 2.33 respectively. This has shown that the greatest change occurs between H and D substitution α e( 14/15N 2D +)/ α e( 14/15N 2H +) ∼ 0.765/0.857 at 300 K and 0.820/0.795 at 500 K respectively. Values at 500 K are consistently larger than at 300 K. 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 500 K, 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.