Crossed molecular beam study of the reactions of methyl bromide with potassium and rubidium

Abstract

Using the crossed molecular beam method, the yields of the alkali halide product MBr from the exoergic reactions CH3Br+M→MBr+CH3 (M≡K, Rb) have been measured as a function of relative translational energy up to 0.9 and 1.3 eV for K and Rb, respectively. Supersonic seeded beams of CH3Br are crossed with thermal alkali beams and the in-plane angular distribution of MBr measured at different average relative translational energies ?tr. The reactions are found to have appreciable energy thresholds, 0.24±0.06 and 0.20±0.06 eV for the K and Rb-reactions, respectively. The product yields increase monotonically with ?tr above threshold. The postthreshold energy dependence of the cross sections has been obtained by deconvoluting these data from the crossed beam velocity distributions. The MBr angular distributions are characteristic of a direct, rebound mechanism, with a large fraction of the available energy going into product translation. The average recoil energy ?′tr of the product MBr increases linearly with ?tr (d?′tr/d?tr?0.73). The present data for the M+CH3Br systems are compared with previous results for the analogous CH3I reactions and with predictions of several theoretical models. The significantly higher activation barriers for the CH3Br reactions account for their smaller thermal reaction rate constants relative to the analogous CH3I reactions (from early flame experiments).