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

Abstract

The chemistry of plasmas is complex, involving many reactive processes such as ionmolecule reactions, ion-ion and electron-ion recombination, and neutral-neutral reactions. Of particular relevance are the plasma processes that take place in interstellar clouds, planetary atmospheres, supernova remnants, cometary comae, etc. The ionization of small neutrals proceeds by multiple gas phase ion-molecule reactions leading to the formation of polyatomic ions which can recombine with electrons, by what is termed dissociative electron-ion recombination (DR). The data presented represents specific examples of ion-molecule chemistry, which leads into studies used to determine the products of DR, both in ground and excited states. Data for ion-molecule chemistry are presented in the reactions of CS2 with a series of fifteen ions (He+, He+ 2 , Ar+, N+ 2 , N+, CO+, CO+ 2 , O+, D+, CS+, C+, S+, CS+ 2 , S+ 2 and D+ 3 ) for which the rate constants and percentage ion product distributions were determined. Dissociative electron-ion recombination (DR) is an important ionization loss process and source of reactive radicals in the interstellar medium (ISM) and many other plasmas. Unfortunately, experimental product distributions are difficult to determine with only about 50 reported in the literature. These have been obtained by spectroscopic techniques integrated with flowing afterglows (FA) and by storage rings (SR). The data obtained by SR measurements are more extensive than those of the FA. Some data are available where the two techniques overlap, however there are very significant discrepancies. 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 neutral products prior to detection by a quadrupole mass filter/electron multiplier tube. 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 quench DR. N2H+ recombination results from this approach give an upper limit of 5% for the NH + N product channel, the remainder being N2 + H. In the second approach, the reagent gas N2 is pulsed. Here the absolute percentages of products were monitored versus initial N2 concentration. Results from this approach give an upper limit of 5% for NH + N production. This establishes that N2 + H is the dominant channel, being at least 95%, and that there is no significant NH production contrary to a recent storage ring measurement in which yielded 64% NH + N and 36% N2 + H. Possible reasons for this dramatic difference are discussed. In addition, DR product distribution of the CH+ 5 system will be discussed. Additionally, excited state products are determined in an emission spectroscopy study of the DR of CS+ 2 and HCS+ 2 . From these studies the DR excited state products are obtained by determining the populated electronic and vibrational states. The relative values of the upper level vibrational state populations are determined for both recombining ions, CS+ 2 and HCS+ 2 .