Imaging the nature of the mode-specific chemistry in the reaction of Cl atom with antisymmetric stretch-excited CH4

Following up our preliminary communication [Kawamata et al., Phys. Chem. Chem. Phys. 10, 4378 (2008)], the effects of the antisymmetric-stretching excitation of methane on the Cl(P-2(3/2)) + CH4 reaction are examined here over a wide range of initial collision energy in a crossed molecular beam imaging experiment. The antisymmetric stretch of CH4 is prepared in a single rovibrational state of (nu(3)=1, j=2) by direct infrared absorption, and the major product states of CH3 (v=0) are probed by a time-sliced velocity-map imaging method. We find that at fixed collision energies, the stretching excitation promotes reaction rate. Compared to the ground-state reaction, this vibrational enhancement factor is, however, no more effective than the translational enhancement. The correlated HCl (v' = 1) vibrational branching fraction shows a striking dependence on collision energies, varying from 0.7 at E-c=2 kcal mol(-1) to about 0.2 at 13 kcal mol(-1). This behavior resembles the previously studied Cl+CH2D2(nu(6) = 1), but is in sharp contrast to the Cl+CHD3(nu(1) = 1) and CH2D2(nu(1) = 1) reactions. Dependences of experimental results on the probed rotational states of CH3(nu = 0) are also elucidated. We qualitatively interpret those experimental observations based on a conceptual framework proposed recently. (C) 2010 American Institute of Physics. [doi:10.1063/1.3482628]