A new technique to study plasma chemistry kinetics

Abstract

Accurate kinetics of plasma processes are necessary for modeling of the chemistry occurring in the upper atmosphere, re-entry, combustion, and discharges. While a great deal of data exists in the literature for most types of plasma chemistry processes, there remain gaps for reactions less amenable to traditional measurements. In particular ion-ion mutual neutralization reactions have received relatively little study, and essentially no detailed product branching fractions are known. Similarly, while hundreds of electron attachment rate constants to stable species have been reported, only a handful of measurements of electron attachment rate constants to unstable radical species exist in the literature. We report several measurements involving these classes of reactions using a novel flowing afterglow technique: Variable Electron and Neutral Density Attachment Mass Spectrometry (VENDAMS). The technique takes advantage of these processes occurring as secondary and tertiary chemistry in high density plasmas, and uses excess electrons as chemical ionization agents to monitor neutral product concentrations. Systems starting with a variety of neutrals have been studied over a temperature range of 300 to 550 K. We report several novel measurements including: the first temperature dependences of electron attachment rate constants to radical species; the first complete neutral product distributions of ion-ion mutual neutralization reactions along with the associated temperature dependences; evidence of a novel plasma chemistry process (A+ + B− + e− neutrals + e−), which we term electron catalyzed mutual neutralization.