Development of a novel technique for quantitatively determining the products of electron-ion dissociative recombination

Abstract

Dissociative electron-ion recombination (DR) is an important ionization loss process and source of neutral reactive radicals in the interstellar medium (ISM) and many other molecular plasmas. Unfortunately, neutral products are difficult to identify with only about 40 distributions being reported in the literature. These have been obtained by spectroscopic techniques integrated with flowing afterglows (FA) and using storage rings (SR). The data obtained by SR measurements are more extensive than those determined in the FA. Some data are available where the two techniques overlap, however here there are very significant differences. To resolve these contradictions, a new technique to quantitatively detect product neutrals has been developed. This technique is based on the FA and uses an electron impact (EI) ionizer to ionize the neutral products prior to detection by a quadrupole mass filter/electron multiplier. Two experimental methodologies, both using pulsed gas techniques, isolate and quantify the DR products. In one approach, an electron attaching gas is pulsed into the flow to transiently destroy electrons and thus quench DR. N 2H + recombination has been used as a test case and results from this approach give an upper limit of 5% for the NH + N product channel, the remainder being N 2 + H. In the second approach, the reagent gas N 2 is pulsed. Here the absolute percentages of products are monitored versus initial N 2 concentration. Results from this approach also give an upper limit of 5% for NH + N production. This establishes that N 2 + H is the dominant channel, being at least 95%, and that there is no significant NH production. This was contrary to a recent storage ring measurement which yielded 64% NH + N and 36% N 2 + H. Note, that these values have changed due to a recent re-measurement of the DR revealing that the NH channel is not nearly as significant as originally thought. Additionally, the DR product distribution for CH 5 + is reported and discussed.