Arrhenius Behavior of Electron Attachment to CH3Br from 303 to 1100 K

Abstract Thermal electron attachment to CH 3 Br has been studied over the temperature range 303–1100 K using two flowing-afterglow Langmuir-probe apparatuses. The reaction yielded only Br − product over this temperature range. The rate coefficient for electron attachment to CH 3 Br was measured to be 8 ± 4 × 10 −12  cm 3  s −1 at 303 K, and was observed to increase strongly with gas temperature. Rate coefficients for the reaction show Arrhenius behavior over the entire temperature range with an activation energy of 260 ± 20 meV. The results are in substantial agreement with earlier data covering a smaller temperature range. Kinetic modeling implies that this behavior and the small rate coefficient at room temperature are due to a barrier in the crossing from the neutral to the anionic potential surfaces of ∼280 meV that dominates other factors in the attachment reaction. There is a hint of the Arrhenius plot reaching saturation at the highest temperatures. While examining an electron-cation recombination correction, the rate coefficient (1.8 ± 0.4 × 10 −9  cm 3  s −1 ) of the reaction Ar +  + CH 3 Br was measured at 302 K, and the ion products identified (80% CH 3 + and 20% CH 2 Br + ). A secondary reaction forming the adduct (CH 3 Br)CH 3 + was seen to occur with a rate coefficient of 2.8 ± 1.0 × 10 −9  cm 3  s −1 .