Product ion distributions and rate coefficients for the attachment reactions of electrons with CHCl 2Br, CCl 2Br 2, and CHClBr 2

Abstract

Attachment rate coefficients, β, and product ion distributions, R, have been determined for the chlorobromomethanes CHCl 2Br, CCl 2Br 2, and CHClBr 2 using the flowing afterglow/Langmuir probe (FALP) method. Thus β and R have been determined at two attaching gas temperatures, T g , of 300 K and 540 K and at electron temperatures, T e , ranging from T g to about 5000 K. The results show that T g rather than T e has the stronger influence on both β and R and that Br − ions are the major products of these three reactions. Both β and Cl − increase markedly with T g , but the variations of β and R with T e , although not so straightforward, are nevertheless well defined. Thus, it is apparent that dissociation to reaction products of the nascent negative ions is efficiently promoted by preheating the attaching molecules (increasing T g ), and to a lesser extent by increasing T e , which is generally in accordance with a model we previously developed to describe other haloalkane attachment reactions. Electron capture by CHCl 2Br and CCl 2Br 2 vary with T e as predicted by capture rate theory, whereas autodetachment apparently seriously decreases the effective capture rate by CHClBr 2. By combining β and R values for these reactions under truly thermalised conditions, the partial rate coefficients for the Br − and Cl − reaction channels have been derived and from their variation with T g , Arrhenius activation energies for Br − and Cl − production have been estimated. Small fractions of dihalogen negative ions are produced in these attachment reactions with ClBr − ions representing a significant fraction of the product ion distribution for the CHClBr 2 reaction.