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

We report on the reactive scattering of methyl iodide, CH 3 I, 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 CH 3 I, 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.