Collisional relaxation of vibrationally excited O2+ ions

Abstract

Vibrationally excited O₂⁺ ions were produced and injected into a helium flow tube and found to survive more than 10⁵ collisions with He atoms without significant relaxation. The first, second, and higher vibrational states of O₂⁺ were detected by their enhanced reactions with Xe, SO₂, and H₂O. Using these reactions as probes for vibrational excitation, the relaxation of O₂⁺ (v=1) and O₂⁺ (v=2) ions was studied for collisions with Ne, Ar, Kr, H₂, D₂, N₂, CO, CO₂, H₂O, CH₄, SO₂, SF₆, and O₂. The resulting quenching rate constants were found to vary from 1(−9) to 3 s⁻¹. The quenching rate constant of O₂⁺ (v=2) was approximately twice that of O₂⁺ (v=1) in each case. The rate constants were found to correlate with the bond energy of the ion–molecule collision complex. For CO₂, Kr, and O₂, the energy dependence of the quenching rate constant was investigated in the range 0.04–0.3 eV. The results indicate that the relaxation process proceeds through a long‐lived complex where the vibrational excitation is transferred into the weak complex bond resulting in dissociation, analogous to vibrational predissociation in van der Waals complexes. In the special case of O₂⁺ (v)–O₂ collisions, the relaxation is clearly dominated by the resonant charge transfer process and the rate constants for v=1 and v=2 are found to be equal.