Low-energy dissociative electron attachment to BrCN and CBrCl3: Temperature dependences and reaction dynamics

Low-energy dissociative electron attachment to BrCN and CBrCl3 over the tem perature range 300-450 K is examined by measuring the velocity and angular distributions of negative ions produced through electron transfer in collis ions with velocity-selected K(np) Rydberg atoms. The data are analyzed usin g a Monte Carlo collision code that models the detailed reaction dynamics. Measurements with BrCN indicate that electron capture leads to the creation of excited BrCN-* intermediates that dissociate with a mean lifetime tau s imilar to 20 ps to form CN- ions, the majority of the excess energy of reac tion appearing in translation. No significant Br- production was observed o ver the present temperature range. Electron transfer to CBrCl3 is found to lead to the formation of both Cl- and Br- ions, the branching ratio dependi ng markedly on temperature. At room temperature, Br- production is dominant , at elevated temperatures Cl- production dominates. The data show that Cl- formation is associated with two reaction channels: in the first the elect ron is captured directly into an antibonding orbital followed by immediate dissociation, in the second a longer-lived CBrCl3-* intermediate is formed with a lifetime greater than or similar to a few vibrational periods. The c ontribution from direct dissociation increases dramatically at the higher t emperatures. Similar reaction channels are operative in the formation of Br - ions, but their relative strengths are not strongly temperature dependent . Possible reaction scenarios are discussed. (C) 2001 American Institute of Physics.