Product ion distributions and rate coefficients for the attachment reactions of electrons with CHCl2Br, CCl2Br2, and CHClBr2

Attachment rate coefficients, beta, and product ion distributions, R, have been determined for the chlorobromomethanes CHCl2Br, CCl2Br2, and CHClBr2 u sing the flowing afterglow/Langmuir probe (FALP) method. Thus beta and R ha ve 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 b eta and R and that Br- ions are the major products of these three reactions . Both beta and Cl- increase markedly with T-g, but the variations of beta and R with T-e, although not so straightforward, are nevertheless well defi ned. Thus, it is apparent that dissociation to reaction products of the nas cent negative ions is efficiently promoted by Dreheating. the attaching mol ecules (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 o ther haloalkane attachment reactions. Electron capture by CHCl2Br and CCl2B r2 vary with T-e as predicted by capture rate theory, whereas autodetachmen t apparently seriously decreases the effective capture rate by CHClBr2. By combining beta and R values for these reactions under truly thermalised con ditions, the partial rate coefficients for the Br- and Cl- reaction channel s have been derived and from their variation with T-g, Arrhenius activation energies for Br- and Cl- production have been estimated. Small fractions o f dihalogen negative ions are produced in these attachment reactions with C lBr- ions representing a significant fraction of the product ion distributi on for the CHClBr2 reaction, (Int J Mass Spectrom 205 (2001) 243-252) (C) 2 001 Elsevier Science B.V.