Reaction dynamics of atomic chlorine with methane: Importance of methane bending and torsional excitation in controlling reactivity

The reactions of atomic chlorine with CH4 and CD4 were studied at five collision energies ranging from 0.13 to 0.29 eV using resonance-enhanced multiphoton ionization of the CH3 and CD3 products. Care-extracted ion arrival profiles were used to determine methyl radical product speed distributions. The distributions contain products that are :moving anomalously fast which energetically cannot result from the reaction of ground-state chlorine with ground-state methane. We attribute these products to reaction of ground-state chlorine with methane vibrationally excited in trace quantities into low-energy bending and torsional modes. Measurements of product spatial anisotropy are used to confirm this interpretation and to indicate that the possible reaction of spin-orbit excited chlorine is less important. These low-energy vibrations create large enhancements in reactivity over ground-state molecules, and consequently, vibrationally excited reagents dominate reactivity at low collision energies and contribute substantially at the highest collision energies studied. It is suggested that vibrationally excited reagents play an important role in the thermal kinetics of the reaction of chlorine with methane and may contribute significantly to explain the observed deviation from Arrhenius equation behavior. Scattering distributions of the products of both ground-state and vibrationally excited reactions are reported, and additional measurements of the internal state distributions of the CH3 and CD3 products reveal that the methyl radicals contain very little energy in rotation or vibration. (C) 1998 American Institute of Physics. [S021-9606(98)01646-8].