Chemical Activation of a Deuterium Molecule by Collision with a Quantum Electronic State-Selected Vanadium Cation

By combining a newly developed spin-orbit electronic state-selected ion source for vanadium cations (V⁺) with a double quadrupole-double octopole mass spectrometer, we have investigated in detail the chemical reactivity or integral cross sections (σ's) for the reactions of V⁺[a⁵D (J = 0, 1), a⁵F (J = 1, 2), and a³F (J = 2, 3)] ion with a deuterium molecule (D₂). The vanadium deuteride ion (VD⁺) is identified to be the only product ion formed in the center-of-mass collision energies of E_cm = 0.1-10.0 V. No J dependence for the σ's is discernible for individual electronic states, indicating that the spin-orbit coupling is weak and has little effect on the chemical reactivity of the titled reaction. The maximum σ value for the V⁺(a³F) state [σ(a³F)] is about 7 and 70 times larger than those for σ(a⁵D) and σ(a⁵F), respectively, showing that the triplet V⁺(a³F) state is dominantly more reactive than the quintet states. Although the V⁺(a⁵F) state is 0.3 eV higher than the V⁺(a⁵D) ground state, the chemical reactivity of the V⁺(a⁵F) state is significantly lower than that of the V⁺(a⁵D) state, clearly indicating that the differences in chemical activity observed are due to quantum electronic states rather than energy effects. The E_cm thresholds determined for σ(a⁵D), σ(a⁵F), and σ(a³F) are consistent with the respective energetics for the formation of VD⁺ from the V⁺(a⁵D, a⁵F, and a³F) + D₂ reactions. The analysis of E_cm threshold measurements yields a bond energy of D₀(V⁺-D) = 2.5 ± 0.2 eV, suggesting that the previously reported values are too low by up to 0.4 eV. The large differences for σ(a⁵D, a⁵F, and a³F) observed here indicate that the activation of D₂ by a V⁺ ion can be efficiently controlled by selecting the V⁺ electronic state as well as the E_cm. The quantum state-selected σ values presented here can also serve as experimental benchmarks for first-principles theoretical reaction dynamics calculations.