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 temperature range 300–450 K is examined by measuring the velocity and angular distributions of negative ions produced through electron transfer in collisions with velocity-selected K(np) Rydberg atoms. The data are analyzed using 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 t;20 ps to form CN ions, the majority of the excess energy of reaction appearing in translation. No significant Br production was observed over the present temperature range. Electron transfer to CBrCl3 is found to lead to the formation of both Cl and Br ions, the branching ratio depending 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 electron is captured directly into an antibonding orbital followed by immediate dissociation, in the second a longer-lived CBrCl3 * intermediate is formed with a lifetime * a few vibrational periods. The contribution from direct dissociation increases dramatically at the higher temperatures. 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. © 2001 American Institute of Physics. @DOI: 10.1063/1.1364686#