Imaging Frontside and Backside Attack in Radical Ion-Molecule Reactive Scattering

We report on the reactive scattering of methyl iodide, CH₃I, with atomic oxygen anions O. This radical ion-molecule reaction can produce different ionic products depending on the angle of attack of the nucleophile O on the target molecule. We present results on the backside and frontside attack of O on CH₃I, which can lead to I and IO products, respectively. We combine crossed-beam velocity map imaging with quantum chemical calculations to unravel the chemical reaction dynamics. Energy-dependent scattering experiments in the range of 0.3-2.0 eV relative collision energy revealed that three different reaction pathways can lead to I products, making it the predominant observed product. Backside attack occurs via a hydrogen-bonded complex with observed indirect, forward, and sideways scattered iodide products. Halide abstraction via frontside attack produces IO, which mainly shows isotropic and backward scattered products at low energies. IO is observed to dissociate further to I + O at a certain energy threshold and favors more direct dynamics at higher collision energies.