Arrhenius behavior of electron attachment to CH3Br from 303 to 1100 K

Abstract

Thermal electron attachment to CH3Br has been studied over the temperature range 303–1100K using two flowing-afterglow Langmuir-probe apparatuses. The reaction yielded only Br− product over this temperature range. The rate coefficient for electron attachment to CH3Br was measured to be 8±4×10−12cm3s−1 at 303K, 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±20meV. 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 ∼280meV 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±0.4×10−9cm3s−1) of the reaction Ar++CH3Br was measured at 302K, and the ion products identified (80% CH3+ and 20% CH2Br+). A secondary reaction forming the adduct (CH3Br)CH3+ was seen to occur with a rate coefficient of 2.8±1.0×10−9cm3s−1.