Chemical activation of oxygen molecule by quantum electronic state selected vanadium cation: observation of spin-orbit state effects

By employing two-colour laser pulsed field ionisation-photoion (PFI-PI) double-quadrupole-double-octopole ion-molecule apparatus, we have examined the absolute integral cross-section (sigma) for the reaction between vanadium cation (V+) and oxygen molecule (O-2), covering the centre-of-mass kinetic energy (E-cm) range of 0.2-10.0 eV. Here, V+ ion was prepared exclusively in its lowest 13 spin-orbit or J-states, a(5)D(J=0-4), a(5)F(J=1-5) , and a(3)F(J=2-4). The formation of VO+ + O is identified as the only product channel for these exothermic reactions. At E-cm = 0.2-5.0 eV, the sigma values for the three electronic states are found to be in the order: sigma(a(3)F(J)) > sigma(a(5)D(J)) > sigma(a(5)F(J)), whereas these cross-sections become nearly identical at E-cm = 5.0-10.0 eV. This observation has been rationalised by collision-mediated non-adiabatic electronic transitions. The J-state effect for the V+(a(3)F(2, 3)) + O-2 reactions are unambiguously identified for the first time for a reaction involving a transition metal cation. This J-state dependent chemical reactivity observed calls for rigorous theoretical interpretation. The fact that chemical reactivity for the a(5)F(J) excited state is lower than that for the a(5)D(J) ground state indicates that the difference in chemical reactivity is originated from quantum-electronic-state instead of energy effects.