Comparing the dynamical effects of symmetric and antisymmetric stretch excitation of methane in the Cl+CH4 reaction

The effects of two nearly isoenergetic C–H stretching motions on the gas-phase reaction of atomic chlorine with methane are examined. First, a 1:4:9 mixture of Cl2, CH4, and He is coexpanded into a vacuum chamber. Then, either the antisymmetric stretch (ν3=3019 cm−1) of CH4 is prepared by direct infrared absorption or the infrared-inactive symmetric stretch (ν1=2917 cm−1) of CH4 is prepared by stimulated Raman pumping. Photolysis of Cl2 at 355 nm generates fast Cl atoms that initiate the reaction with a collision energy of 1290±175 cm−1 (0.16±0.02 eV). Finally, the nascent HCl or CH3 products are detected state-specifically via resonance enhanced multiphoton ionization and separated by mass in a time-of-flight spectrometer. We find that the rovibrational distributions and state-selected differential cross sections of the HCl and CH3 products from the two vibrationally excited reactions are nearly indistinguishable. Although Yoon et al. J. Chem. Phys. 119, 9568 (2003) report that the reactivities of these two different types of vibrational excitation are quite different, the present results indicate that the reactions of symmetric-stretch excited or antisymmetric-stretch excited methane with atomic chlorine follow closely related product pathways. Approximately 37% of the reaction products are formed in HCl(v=1,J) states with little rotational excitation. At low J states these products are sharply forward scattered, but become almost equally forward and backward scattered at higher J states. The remaining reaction products are formed in HCl(v=0,J) and have more rotational excitation. The HCl(v=0,J) products are predominantly back and side scattered. Measurements of the CH3 products indicate production of a non-negligible amount of umbrella bend excited methyl radicals primarily in coincidence with the HCl(v=0,J) products. The data are consistent with a model in which the impact parameter governs the scattering dynamics.