Imaging the Mode-Specificity in Cl + CH3D(v1-I, v1-II, v4 = 1; |jK⟩ = |10⟩) → CH2D(41) + HCl(v).

Abstract

We report a crossed-beam imaging experiment on the title reactions at two collisional energies (Ec) 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 CH3-stretching vibrations of the CH3D reagent exerted profound mode-specificity in forming the umbrella-mode-excited CH2D(41) products with the vibrational efficacy of v4 > v1-I > v1-II at both Ec 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 Ec = 5.3 kcal mol-1, yet exhibited a pronounced mode-specific dependency in the order of v1-II > v1-I > v4 at Ec = 10.3 kcal mol-1. This large and Ec-dependent disparity between the two Fermi-coupled reagents, v1-I and v1-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 (vCH2D, vHCl)s = (41, 0)s in the three stretch-excited reactions were globally alike and resembled that of the ground-state reaction pair (00, 0)g, suggestive of a direct abstraction mechanism of the peripheral type. This is in sharp contrast to all other vibrationally excited pairs of (11, 0)s, (31, 0)s, and (61, 0)s previously reported in the CH2D + HCl isotopic channel, for which both the direct abstraction and a time-delayed resonance pathway partake.