Ionic Collision Processes in Gaseous Ammonia

Abstract

Rate coefficients for reactions resulting from collisions of NH3+ and NH2+ with NH3 have been measured as a function of primary ion translational energy over the energy range 0–8.0 eV. NH4+ was the only observed product of NH3+/NH3 collisions. The rate coefficient drops sharply with increasing energy from 18.1 × 10−10 cm3 molecule−1·sec−1 for thermal ions to 10.2 × 10−10 cm3 molecule−1·sec−1 for ions with a maximum translational energy (KE) of 1 eV. Above 3 eV an essentially constant value of 8 × 10−10 cm3 molecule−1·sec−1 is obtained. Collisions of NH2+/NH3 result in the production of both NH4+ and NH3+. With increasing ion energy below 1 eV the relative number of collisions which proceed via the less exoergic mechanism increases. Above 3-eV KE the rate coefficient for the removal of NH2+ increases which is taken to indicate that in this energy range the cross section for electron transfer is greater than that for ion–molecule reaction. The rate coefficient for NH4+ production drops to an undetectable level at about 2-eV KE rising again above 3-eV KE. It is proposed that this new onset can be attributed to the endoergic reaction in which the neutral products are N+H and not NH as is supposed for the proton-transfer process at lower ion energies.