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 (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-modeexcited 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.

Stretching-mode specificity in the Cl + CH3D(v1-I, v1-II, and v4 = 1; |jK〉) reactions: dependency on the initial |jK〉 selectivity

“…The crossed-beam set-up and experimental details are essentially the same as that described in previous reports. [1][2][3][4]35,36 Briefly, two doubly skimmed, pulsed molecular beams intersected in the interaction region of a time-sliced, velocity-map imaging apparatus. After the reaction, the CH 2 D(0 0 /6 1 ) products were detected by a (2 + 1) REMPI scheme near 333.7 nm via the intermediate 3p 2 B 1 Rydberg state, 1,3,12 and the REMPItagged ion signals were recorded using a time-sliced, velocity-map imaging technique.…”

“…Over the past few years, our laboratory at IAMS has carried out an extensive study on the effects of the CH 3 -stretching excitations of CH 3 D(v i = 1) in reactions with the Cl-atom. [1][2][3][4] The choice of the Cl + CH 3 D(v i ) reaction for such comprehensive investigations is because it offers the opportunity to address several fundamentally important aspects of polyatomic reactivity in general. [1][2][3][4][5][6][7][8][9][10] From the reagent side, there are three different CH 3 -stretches (v i ): the degenerate mode of asymmetric stretch (v 4 ) and a pair of Fermi-coupled symmetric-stretch/ overtone-bend states (v 1 -I and v 1 -II).…”

“…[1][2][3][4] The choice of the Cl + CH 3 D(v i ) reaction for such comprehensive investigations is because it offers the opportunity to address several fundamentally important aspects of polyatomic reactivity in general. [1][2][3][4][5][6][7][8][9][10] From the reagent side, there are three different CH 3 -stretches (v i ): the degenerate mode of asymmetric stretch (v 4 ) and a pair of Fermi-coupled symmetric-stretch/ overtone-bend states (v 1 -I and v 1 -II). 7,11 Hence, one can address not only the impacts of vibrational-mode specificity on chemical reactivity (v 4 versus v 1 ), 7 but also the possible influence of the quantum mechanical (Fermi) phase on the different reactivities between the v 1 -I and v 1 -II reagents.…”

“…7,11 Hence, one can address not only the impacts of vibrational-mode specificity on chemical reactivity (v 4 versus v 1 ), 7 but also the possible influence of the quantum mechanical (Fermi) phase on the different reactivities between the v 1 -I and v 1 -II reagents. 1,2 From the product side, two isotopic channels of CH 3 + DCl and CH 2 D + HCl are possible. In addition, both methyl radical products possess multiple vibrational modes, for which the state-specific detection using a (2 + 1) resonance-enhanced multiphoton ionization (REMPI) scheme for all of them has recently been established.…”

“…Crucially, all studies outlined above employed the same single rotational state of |jKi = |10i for the vibrationally excited reagents CH 3 D(v 4 , v 1 -I, and v 1 -II). [1][2][3][4] As such, the reactivity comparison among different stretch-excited reactions could be free of (or at least with minimal) rotational-mode effects. Nonetheless, previous studies on the two analogous reactions of Cl + CHD 3 (v 1 = 1, |jKi) and CH 4 (v 3 = 1, |jNli) did reveal prominent, perplexing rotational-mode impacts on both ICS and DCS.…”