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

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 (sigma's) for the reactions, V+[a(5)D(J) (J = 0, 2), a(5)F(J) (J = 1, 2), and a(3)F(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+ + H-2, and VCH3+ + H, are unambiguously identified based on E-cm-threshold measurements. No J-dependences for the sigma curves (sigma 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 sigma values for the triplet a(3)F(J) state [sigma(a(3)F(J))] are found to be more than ten times larger than those for the quintet sigma(a(5)D(J)) and sigma(a(5)F(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 sigma(a(5)D(0), a(5)F(1), and a(3)F(2)) 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: D-0(V+-H) = 2.02 (0.05) eV, D-0(V+-CH2) = 3.40 (0.07) eV, and D-0(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 sigma curves obtained previously for V+ generated by surface ionization at 1800-2200 K can be achieved by the linear combination of the sigma(a(5)D(J), a(5)F(J), and a(3)F(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+(a(5)D(J), a(5)F(J), and a(3)F(J)) states prepared in this experiment are in single spin-orbit states with 100% purity.