Quantum-state-selected integral cross sections for the charge transfer collision of O2+(a4Πu5/2,3/2,1/2,−1/2: v+ = 1–2; J+) [O2+(X2Πg3/2,1/2: v+ = 22–23; J+)] + Ar at center-of-mass collision energies of 0.05–10.00 eV

Abstract

By employing the sequential electric field pulsing scheme for vacuum ultraviolet (VUV) laser pulsed field ionization-photoion (PFI-PI) detection, we have successfully recorded the spin–orbit and rovibronic state resolved VUV-PFI-PI spectra for O₂⁺(a⁴Π_{u5/2,3/2,1/2,−1/2}: ν⁺ = 0–2; J⁺) and O₂⁺(X²Π_g3/2,1/2: ν⁺ = 21–23; J⁺), indicating that O₂⁺(a⁴Π_u) and O₂⁺(X²Π_g) ions in these spin–orbit and rovibronic states can be prepared for ion–molecule collision studies. The present experiment is concerned with the measurement of absolute integral cross sections (σ's) of the charge transfer reactions, O₂⁺(a⁴Π_{u5/2,3/2,1/2,−1/2}: ν⁺ = 1, 2; J⁺) [O₂⁺(X²Π_g1/2,3/2: ν⁺ = 22, 23)] + Ar → Ar⁺ + O₂. The fact that the O₂⁺(a⁴Π_{u5/2,3/2,1/2,−1/2}: ν⁺ = 1) and O₂⁺(X²Π_g3/2,1/2: ν⁺ = 22) [O₂⁺(a⁴Π_{u5/2,3/2,1/2,−1/2}: ν⁺ = 2) and O₂⁺(X²Π_g3/2,1/2: ν⁺ = 23)] states are in close energy resonance, makes these reactions ideal model systems for investigating the energy resonance and Franck–Condon factor (FCF) effects on the charge transfer reactivity of O₂⁺. The σ(a⁴Π_{u5/2,3/2,1/2,−1/2}: ν⁺ = 1, 2) values are found to be about ten-fold higher than the σ(X²Π_g3/2,1/2: ν⁺ = 22, 23) values at E_cm = 0.05–10.00 eV, indicating that the FCFs play a predominant role in promoting these charge transfer reactions. The present ion–molecule reaction study also shows that σ(a⁴Π_u) depends strongly on the spin–orbit as well as the vibrational states with the order: σ(a⁴Π_u: v⁺ = 2) > σ(a⁴Π_u: v⁺ = 1), and σ(a⁴Π_u5/2: v⁺) > σ(a⁴Π_u3/2: v⁺) > σ(a⁴Π_u1/2: v⁺) > σ(a⁴Π_u−1/2: v⁺), where v⁺ = 1 and 2. The high σ(a⁴Π_{u5/2,3/2,1/2,−1/2}: v⁺ = 1, 2) values, along with their decreasing trend with increasing E_cm, are consistent with those expected for a long range charge transfer mechanism. However, the low σ(X²Π_g3/2,1/2: ν⁺ = 22, 23) values and the lack of E_cm-dependence observed in the E_cm range of 0.05–10.00 eV point to the involvement of short-range collision dynamics.