Effects of C-H stretch excitation on the H+CH4 reaction.

We have investigated the effects of C-H stretching excitation on the H+CH4-->CH3+H2 reaction dynamics using the photo-LOC technique. The CH3 product vibrational state and angular distribution are measured for the reaction of fast H atoms with methane excited in either the antisymmetric stretching fundamental (nu3=1) or first overtone (nu3=2) with a center-of-mass collision energy of Ecoll ranging from 1.52 to 2.20 eV. We find that vibrational excitation of the nu3=1 mode enhances the overall reaction cross section by a factor of 3.0+/-1.5 for Ecoll=1.52 eV, and this enhancement factor is approximately constant over the 1.52-2.20-eV collision energy range. A local-mode description of the CH4 stretching vibration, in which the C-H oscillators are uncoupled, is used to describe the observed state distributions. In this model, the interaction of the incident H atom with either a stretched or an unstretched C-H oscillator determines the vibrational state of the CH3 product. We also compare these results to the similar quantities obtained previously for the Cl+CH4-->CH3+HCl reaction at Ecoll=0.16 eV [Z. H. Kim, H. A. Bechtel, and R. N. Zare, J. Chem. Phys. 117, 3232 (2002); H. A. Bechtel, J. P. Camden, D. J. A. Brown, and R. N. Zare, ibid. 120, 5096 (2004)] in an attempt to elucidate the differences in reactivity for the same initially prepared vibration.