Imaging the Mode-Specificity in Cl + CH 3 D( v 1 -I, v 1 -II, v 4 = 1; | jK ⟩ = |10⟩) → CH 2 D(4 1 ) + HCl( v ).

We report a crossed-beam imaging experiment on the title reactions at two collisional energies ( E c ) of 5.3 and 10 kcal mol -1 . Both the integral cross sections relative to the ground-state reactivity and the differential cross sections were measured and compared. We found that one-quantum excitations of the CH 3 -stretching vibrations of the CH 3 D reagent exerted profound mode-specificity in forming the umbrella-mode-excited CH 2 D(4 1 ) products with the vibrational efficacy of v 4 > v 1 -I > v 1 -II at both E c values. The concomitantly formed HCl( v ) coproducts were vibrationally cold. Interestingly, the branching ratios of ( v = 1)/( v = 0) appeared invariant to the initial stretch-modes of excitation at E c = 5.3 kcal mol -1 , yet exhibited a pronounced mode-specific dependency in the order of v 1 -II > v 1 -I > v 4 at E c = 10.3 kcal mol -1 . This large and E c -dependent disparity between the two Fermi-coupled reagents, v 1 -I and v 1 -II, is particularly significant and could be another facet─in addition to that in the recently reported vibrational enhancement factors─of the Fermi-phase-induced interference effect manifested in the product vibrational branching ratio. The pair-correlated angular distributions ( v CH2D , v HCl ) s = (4 1 , 0) s in the three stretch-excited reactions were globally alike and resembled that of the ground-state reaction pair (0 0 , 0) g , suggestive of a direct abstraction mechanism of the peripheral type. This is in sharp contrast to all other vibrationally excited pairs of (1 1 , 0) s , (3 1 , 0) s , and (6 1 , 0) s previously reported in the CH 2 D + HCl isotopic channel, for which both the direct abstraction and a time-delayed resonance pathway partake.