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Velocity map imaging of the dynamics of the CH3 + HCl → CH4 + Cl reaction using a dual molecular beam method

The reactions CH3 + HCl → CH4 + Cl(2P3/2) and CD3 + HCl → CD3H + Cl(2P3/2) have been studied by photo-initiation (by CH3I or CD3I photolysis at 266 nm) in a dual molecular beam apparatus. Product Cl(2P3/2) atoms were detected using resonance enhanced multi-photon ionisation and velocity map imaging, revealing product translational energy and angular scattering distributions in the centre-of-mass frame. Image analysis is complicated by the bimodal speed distribution of CH3 (and CD3) radicals formed in coincidence with I(2P3/2) and I(2P1/2) atoms from CH3I (CD3I) photodissociation, giving overlapping Newton diagrams with displaced centre of mass velocities. The relative reactivities to form Cl atoms are greater for the slower CH3 speed group than the faster group by factors of ∼1.5 for the reaction of CH3 and ∼2.5 for the reaction of CD3, consistent with the greater propensity of the faster methyl radicals to undergo electronically adiabatic reactions to form Cl(2P1/2). The average fraction of the available energy becoming product translational energy is ⟨ft ⟩ = 0.48 ± 0.05 and 0.50 ± 0.03 for reaction of the faster and slower sets of CH3 radicals, respectively. The Cl atoms are deduced to be preferentially forward scattered with respect to the HCl reagents, but the angular distributions from the dual beam imaging experiments require correction for under-detection of forward scattered Cl products. A correction function is deduced from separate measurements on the Cl + C2H6 reaction, for which the outcomes can be compared with published differential cross-sections from crossed molecular beam experiments. Monte Carlo simulations of the dual beam experimental method suggest that the source of the depletion is secondary collisions of the slowest moving reaction products (in the laboratory frame) with unreacted reagents or carrier gas in one of the molecular beams.