Effects of Stretching Excitations in Cl + CH3D( v4, v1-I, v1-II = 1): As the Cl Atom Attacks the Unexcited C-D Bond.

The title reactions were studied at two collisional energies ( Ec) in a crossed-beam product-imaging experiment. We found that all three initial CH stretching excitations suppress the reactivity toward the abstraction of the unexcited D atom. In terms of vibrational suppression factor, σs/σg, the product channels of CH3(00/41) + DCl and CH3(11/31) + DCl show opposite mode-specific trends. However, the angular distributions of both channels are nearly identical to that of the ground-state reaction at the same Ec, regardless of the initial reactant states. Tentatively, we ascribed these two observations to a vibrationally induced narrowing effect of the attack angle near the barrier to reaction. As for the DCl coproduct state distributions, the two channels are distinct but show little mode-specific difference. When CH3(00) is probed, the DCl coproduct peaks at v = 1 are accompanied by substantial rotational excitation, whereas the DCl products associated with CH3(11/31) are both vibrationally and rotationally cold. We attributed the different (correlated) energy disposals to a manifestation of trajectory bifurcations in the postbarrier region, with a vibrationally nonadiabatic pathway leading to CH3(00) + DCl( v = 1) and the other adiabatically to the CH3(11/31) + DCl( v = 0) channel. For both pathways the initial CH stretching excitation acts as a conserved mode by preferentially retaining one quantum of vibrational excitation in the reaction.