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

Abstract

Branching ratios and absolute cross sections have been measured for the dissociative recombination of N 2 H + using the CRYRING ion storage ring. It has been found that the channel N 2 H + + e À ! N 2 + H accounts for only 36% of the total reaction and that the branching into the other exoergic pathway, N 2 H + + e À ! NH + N, consequently amounts to 64%. The cross section of the reaction could be fitted by the expression ¼ (2:4 AE 0:4) ; 10 À16 E À1:04AE0:02 cm 2 , which leads to a thermal reaction rate of k(T) ¼ (1:0 AE 0:2) ; 10 À7 (T =300) À0:51AE0:02 cm 3 s À1 , 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 N 2 H + in the dark clouds but a five-to sevenfold increase of the NH concentration as steady state is reached.

Nitrogen hydrides in the cold envelope of IRAS 16293-2422

“…Figure A.2 shows the o:p ratio for NH 2 assuming equilibrium at a single temperature for all levels. dissociative recombination (DR) of the nitrogen bearing cations N 2 H + , NH + 2 , and NH + 4 : -The DR of N 2 H + has been determined experimentally using both flowing afterglow (FA, see Adams et al 1991) and storage ring (SR) techniques (Geppert et al 2004), leading to controversial results concerning the branching ratios (BR) of the two channels N 2 + H and NH + N. Thus, in contrast to the FA results that established the major product as N 2 + H with a BR ≈100%, Geppert et al (2004) found this channel to account for only 36% of the total reaction. The most recent FA and SR measurements (Molek et al 2007;Adams et al 2009), however, have confirmed the earlier FA results that the DR of N 2 H + should lead predominantly to N 2 + H with a BR ≈90−100%.…”

“…For the total rate Molek et al (2007) , (c) Thomas et al (2005) , (d) Öjekull et al (2004). coefficient, we adopted the (temperature dependent) expression of Geppert et al (2004), as in the osu.09.2008 network.…”

Using block diagonalization to determine dissociating autoionizing states: Application to N2H, and the outlook for SH

“…The measurements of Adams et al [4] in 1990 indicated that the branching ratios in the dissociative recombination process strongly favor the N 2 + H channel. However, in 2004 Geppert et al [5] reported measurements that the branching ratio to the NH + N channel was dominant. This unexpected result cast doubt on the assumptions mentioned above and motivated additional experimental and theoretical work.…”

Observations of chemical differentiation in clumpy molecular clouds

“…17]. The chemical model is based on the model of Rodgers & Charnley [60], and has been updated to incorporate recent laboratory measurements of the dissociative recombination channels for several ions, including CH 3 OH + 2 [44,61,62,63,64]. The initial molecular abundances in the newly-formed clump are determined by those in the interclump medium, which are themselves controlled by both (i) the chemistry in these regions, and (ii) whether the material previously passed through a dense phase.…”