Mutual neutralization of simple and clustered positive and negative ions

Abstract

Measurements are reported of the rate coefficients, α, for several ion–ion mutual neutralization reactions principally involving NH4 + ions and their ammonia clusters NH4 +⋅ (NH3)1,2,3 with several different negative ions. The data were obtained utilizing an ion–ion flowing afterglow plasma combined with Langmuir probediagnostics. Most of the measurements were obtained at 300 K although the NH4 ++Cl−reaction has also been studied at 220 and 430 K. Both the absolute magnitude of α and its temperature variation are shown to be in acceptable agreement with theoretical predictions. The α for the cluster ion reactions are very similar to those for the simpler ions, even when both positive and negative ions are large clusters, although an increase in the mean reaction cross section for the cluster ion reactions is discernible. All the α measured to date for both simple and cluster ion reactions, albeit for species of high electron affinity and over the limited temperature range of 180 to 530 K, are within the range (4–10) ×10−8 cm3 s−1, more marked variations occurring with temperature than with ionic mass.