Quantum electronic control on chemical activation of methane by collision with spin–orbit state selected vanadium cation

Abstract

By coupling a newly developed quantum-electronic-state-selected supersonically cooled vanadium cation (V⁺) beam source with a double quadrupole-double octopole (DQDO) ion–molecule reaction apparatus, we have investigated detailed absolute integral cross sections (σ's) for the reactions, V⁺[a⁵D_J (J = 0, 2), a⁵F_J (J = 1, 2), and a³F_J (J = 2, 3)] + CH₄, covering the center-of-mass collision energy range of E_cm = 0.1–10.0 eV. Three product channels, VH⁺ + CH₃, VCH₂⁺ + H₂, and VCH₃⁺ + H, are unambiguously identified based on E_cm-threshold measurements. No J-dependences for the σ curves (σ versus E_cm plots) of individual electronic states are discernible, which may indicate that the spin–orbit coupling is weak and has little effect on chemical reactivity. For all three product channels, the maximum σ values for the triplet a³F_J state [σ(a³F_J)] are found to be more than ten times larger than those for the quintet σ(a⁵D_J) and σ(a⁵F_J) states, showing that a reaction mechanism favoring the conservation of total electron spin. Without performing a detailed theoretical study, we have tentatively interpreted that a weak quintet-to-triplet spin crossing is operative for the activation reaction. The σ(a⁵D₀, a⁵F₁, and a³F₂₎ measurements for the VH⁺, VCH₂⁺, and VCH₃⁺ product ion channels along with accounting of the kinetic energy distribution due to the thermal broadening effect for CH₄ have allowed the determination of the 0 K bond dissociation energies: D₀(V⁺–H) = 2.02 (0.05) eV, D₀(V⁺–CH₂) = 3.40 (0.07) eV, and D₀(V⁺–CH₃) = 2.07 (0.09) eV. Detailed branching ratios of product ion channels for the titled reaction have also been reported. Excellent simulations of the σ curves obtained previously for V⁺ generated by surface ionization at 1800–2200 K can be achieved by the linear combination of the σ(a⁵D_J, a⁵F_J, and a³F_J) curves weighted by the corresponding Boltzmann populations of the electronic states. In addition to serving as a strong validation of the thermal equilibrium assumption for the populations of the V⁺ electronic states in the hot filament ionization source, the agreement between these results also confirmed that the V⁺(a⁵D_J, a⁵F_J, and a³F_J) states prepared in this experiment are in single spin–orbit states with 100% purity.