Dissociative recombination of NH4+ and ND4+ ions: storage ring experiments and ab initio molecular dynamics

J Ojekull; P U Andersson; M B Någård; J B C Pettersson; A M Derkatch; A Neau; S Rosén; R Thomas; M Larsson; F Osterdahl; J Semaniak; H Danared; A Källberg; M af Ugglas; N Marković

doi:10.1063/1.1669388

Dissociative recombination of NH4+ and ND4+ ions: storage ring experiments and ab initio molecular dynamics

J Chem Phys. 2004 Apr 22;120(16):7391-9. doi: 10.1063/1.1669388.

Authors

J Ojekull¹, P U Andersson, M B Någård, J B C Pettersson, A M Derkatch, A Neau, S Rosén, R Thomas, M Larsson, F Osterdahl, J Semaniak, H Danared, A Källberg, M af Ugglas, N Marković

Affiliation

¹ Department of Chemistry, Atmospheric Science, Göteborg University, SE-412 96 Göteborg, Sweden.

PMID: 15267649
DOI: 10.1063/1.1669388

Abstract

The dissociative recombination (DR) process of NH4+ and ND4+ molecular ions with free electrons has been studied at the heavy-ion storage ring CRYRING (Manne Siegbahn Laboratory, Stockholm University). The absolute cross sections for DR of NH4+ and ND4+ in the collision energy range 0.001-1 eV are reported, and thermal rate coefficients for the temperature interval from 10 to 2000 K are calculated from the experimental data. The absolute cross section for NH4+ agrees well with earlier work and is about a factor of 2 larger than the cross section for ND4+. The dissociative recombination of NH4+ is dominated by the product channels NH3+H (0.85+/-0.04) and NH2+2H (0.13+/-0.01), while the DR of ND4+ mainly results in ND3+D (0.94+/-0.03). Ab initio direct dynamics simulations, based on the assumption that the dissociation dynamics is governed by the neutral ground-state potential energy surface, suggest that the primary product formed in the DR process is NH3+H. The ejection of the H atom is direct and leaves the NH3 molecule highly vibrationally excited. A fraction of the excited ammonia molecules may subsequently undergo secondary fragmentation forming NH2+H. It is concluded that the model results are consistent with gross features of the experimental results, including the sensitivity of the branching ratio for the three-body channel NH2+2H to isotopic exchange.