Direct infrared absorption prepares CH4 in two nearly isoenergetic vibrationally excited states, the symmetric stretch–bend combination (ν1+ν4) and the antisymmetric stretch–bend combination (ν3+ν4), for a study of the effect of stretching vibrations of CH4 on the reaction, CH4+Cl(2P3/2)→CH3+HCl. Comparison of intensities in the action spectra with those in the simulated absorption spectra shows that vibrational excitation of methane to the ν1+ν4 state promotes the reaction more efficiently than excitation to the ν3+ν4 state by a factor of 1.9±0.5. The reaction of methane in both vibrational states produces a substantial fraction (35%) of the CH3 products with the umbrella mode (ν2) excited, which we attribute to the presence of the bending vibration (ν4) in the combination states. The similarity of the vibrational population distributions of the products for the two excitations implies that the differences in the action spectra arise from the relative reactivity of the states rather than from population of different product states. This result is consistent with theoretical calculations that predict a stronger coupling of the symmetric stretching vibration to the reaction coordinate than the antisymmetric stretch. Analyzing the infrared laser power dependence of the signal with a simple two state model shows that the reaction cross section of the ν1+ν4 state of methane is 19±5 times larger than that of methane molecules in their ground or thermally populated vibrational states.

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