State selected ion–molecule reactions by a TESICO technique. VII. Isotope effect in the reactions O2+(X 2Πg,a 4Πu)+HD→O2 H+(O2D+)+D(H)

Abstract

The kinetic isotope effect in the reactions O2++HD→O2H+(O2D+) +D(H) (ΔE=1.78 eV) has been studied by selecting the vibronic states of O2+ using the threshold electron-secondary ion coincidence (TESICO) technique. Relative cross sections for the individual vibronic states, v=19 and 20 of the X 2Πg state and v=0–4 of the a 4Πu state of O2+, have been determined at a collision energy of 0.4 eV. The results for the a 4Πu state show that, regardless of the vibrational state, the isotope effect is consistent with that expected from a direct reaction mechanism. A similar isotope effect has also been observed for v=19 and 20 of the X 2Πg state, suggesting that the above reaction with O2+(X 2Πg) proceeds via a direct mechanism even at collision energies below the translational threshold when a sufficient vibrational energy is supplied to the reactant ion.