Imaging the nonadiabatic dynamics of the CH3 + HCl reaction

Abstract

LAB-frame velocity distributions of Cl-atoms produced in the photoinitiated reaction of CH₃ radicals with HCl have been measured for both the ground Cl (²P_3/2) and excited Cl* (²P_1/2) spin–orbit states using a DC slice velocity-map ion imaging technique. The similarity of these distributions, as well as the average internal excitation of methane co-products for both Cl and Cl* pathways, suggest that all the reactive flux proceeds through the same transition state on the ground potential energy surface (PES) and that the couplings which promote nonadiabatic transitions to the excited PES correlating to Cl* occur later in the exit channel, beyond the TS region. The nature of these couplings is discussed in light of initial vibrational excitation of CH₃ radicals as well as previously reported nonadiabatic reactivity in other polyatomic molecule reactions. Furthermore, the scattering of the reaction products, derived using the photoloc method, suggests that at the high collision energy of our experiment (〈E_coll〉 = 22.3 kcal mol⁻¹), large impact parameter collisions are favoured with a reduced kinematic constraint on the internal excitation of the methane co-product.