Imaging pair-correlated reaction cross sections in F + CH3D(νb = 0, 1) → CH2D(ν4 = 1) + HF(ν)

Abstract

The title reactions were studied in a crossed-beam experiment at collisional energies (E_c) from 0.5 to 4.7 kcal mol⁻¹. The ν_b (ν₄) vibrational mode denotes the bending (umbrella) motion of the CH₃D reactant (CH₂D product). Using a time-sliced, velocity-map imaging technique, we extracted the state-specific, pair-correlated integral and differential cross sections. As with other isotopically analogous ground-state reactions, an inverted vibrational population of the HF coproduct was observed. Both the step-like excitation function near the threshold and the oscillatory forward–backward peakings in the E_c-evolution of the two dominant pair-correlated angular distributions at lower E_c suggest a resonance-mediated, time-delay mechanism. As E_c increases, the angular distribution of the HF(ν = 2) product evolves into a smooth and broad swath in the backward hemisphere, indicative of a direct rebound mechanism. One quantum excitation of the bending modes of CH₃D(ν_b = 1) promotes the reaction rate by two- to three-fold up to E_c = 2.1 kcal mol⁻¹. Broadly speaking, all major findings are qualitatively in line with previous results in the reactions of the F atom with other isotopologues. However, the rainbow feature recently observed in the CH₂D(0₀) + HF(ν = 3) product channel is entirely absent. A possible rationale is put forward, which reinforces the previous reactive rainbow conjecture and calls for future theoretical investigations.