LOW-ENERGY DISSOCIATIVE ELECTRON-ATTACHMENT TO CH2BR2, CHCLBR2, AND CCL3BR - INTERMEDIATE LIFETIMES AND DECAY ENERGETICS

Dissociative electron attachment to the brominated methanes CH2Br2, CH ClBr2, and CCl3Br is investigated by measuring the velocity and angula r distributions of negative ions produced through electron transfer in collisions with velocity selected K(np) Rydberg atoms. The data are a nalyzed with the aid of a Monte Carlo collision code that models the d etailed kinematics of the reactions. Measurements with CH2Br2 show tha t essentially all the excess energy of reaction appears in translation , indicating that the electron is captured directly into an antibondin g orbital. Data for CHClBr2 show that reaction proceeds by two channel s: in the first, essentially all the excess energy appears in translat ion; in the second, limited conversion of the excess energy to interna l motions occurs, pointing to a CHClBr2- intermediate lifetime of sim ilar to 0.1 to 1 ps. In the case of CCl3Br, electron transfer leads to the formation of both Br- and Cl- ions. Br- production is again assoc iated with two channels: in one, essentially all the excess energy app ears in translation; in the other, near statistical redistribution of the excess energy occurs prior to dissociation, pointing to a relative ly long-lived (tau similar to several ps) CCl3Br- intermediate. Data for Cl- production suggest only limited energy transfer to internal mo tions. Possible reaction mechanisms for the various channels are discu ssed with the aid of ab initio calculations. (C) 1998 American Institu te of Physics. [S0021-9606(98)00144-5].