Product ion distributions and rate coefficients for the attachment reactions of electrons with CHCl_2Br, CCl_2Br_2, and CHClBr_2

摘要

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 apprarently 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.