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 coll ision energies ranging from 0.13 to 0.29 eV using resonance-enhanced multip hoton ionization of the CH3 and CD3 products. Care-extracted ion arrival pr ofiles were used to determine methyl radical product speed distributions. T he distributions contain products that are :moving anomalously fast which e nergetically cannot result from the reaction of ground-state chlorine with ground-state methane. We attribute these products to reaction of ground-sta te chlorine with methane vibrationally excited in trace quantities into low -energy bending and torsional modes. Measurements of product spatial anisot ropy are used to confirm this interpretation and to indicate that the possi ble reaction of spin-orbit excited chlorine is less important. These low-en ergy vibrations create large enhancements in reactivity over ground-state m olecules, and consequently, vibrationally excited reagents dominate reactiv ity at low collision energies and contribute substantially at the highest c ollision energies studied. It is suggested that vibrationally excited reage nts play an important role in the thermal kinetics of the reaction of chlor ine with methane and may contribute significantly to explain the observed d eviation from Arrhenius equation behavior. Scattering distributions of the products of both ground-state and vibrationally excited reactions are repor ted, and additional measurements of the internal state distributions of the CH3 and CD3 products reveal that the methyl radicals contain very little e nergy in rotation or vibration. (C) 1998 American Institute of Physics. [S0 21-9606(98)01646-8].