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

Abstract

The reactions CH₃ + HCl → CH₄ + Cl(²P_3/2) and CD₃ + HCl → CD₃H + Cl(²P_3/2) have been studied by photo-initiation (by CH₃I or CD₃I photolysis at 266 nm) in a dual molecular beam apparatus. Product Cl(²P_3/2) 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(²P_3/2) and I(²P_1/2) 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(²P_1/2). The average fraction of the available energy becoming product translational energy is ⟨f_t ⟩ = 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.

Keywords:

• reaction dynamics
• velocity map imaging
• resonance enhanced multiphoton ionisation

Acknowledgements

We are grateful to EPSRC for financial support via the EP/G00224X Programme Grant and to the University of Bristol for a Postgraduate Scholarship (R.A.R.). S.J.G. thanks the Leverhulme Trust for an Early Career Research Fellowship and A.J.O.E. is grateful to the Royal Society and the Wolfson Foundation for a Research Merit Award. We thank Dr C.S. Huang and Prof. A.G. Suits (Wayne State University) for providing us with the crossed-beam data for the Cl + ethane reaction.