A study of the ion–molecule half reactions O+2(ã4Πu, v)⋅⋅⋅(O2)m→O+2m+1+O, m = 1, 2, or 3, using the molecular beam photoionization method

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Abstract
The photoionization efficiency (PIE) curve for (O2)+2 has been obtained in the region 650–1 080 Å using the molecular beam photoionization method. The ionization energy (IE) for (O2)2 is determined to be 11.66±0.03 eV. From the measured IE for (O2)+2, the known IE for O2, and the estimated dissociation energy (0.01 eV) of (O2)2, the binding energy for (O2)+2 is deduced to be 0.42±0.03 eV. Comparisons of the PIE spectra for O+2 and (O2)+m, where m = 2, 3, and 4 indicate that the excited dimer complexes O*2(n,v)⋅(O2)m (m = 1, 2, and 3) formed in this wavelength region are almost completely dissociative, and the cluster ions are predominately formed by the direct photoionization processes (O2)m = 2, 3, or 4+hn→(O2)+m = 2, 3, or 4+e. The PIE curves for O+3, O+5, and O+7 are measured in the region 650–780 Å. The appearance energy 16.66±0.03 eV (744±1.5 Å) for O+3 is found to be consistent with a zero activation energy for the ion‐molecule reaction O+2(X̃2Rg)+O2→O+3+O. The appearance energy for O+5 is determined to be 16.41±0.06 eV (755.5±3 Å). This value has allowed the determination of a binding energy of 0.26 eV for O+3⋅O2. The nearly structureless PIE spectra observed for O+3, O+5, and O+7 also suggests that these ions originate mainly from (O2)+2, (O2)+3, and (O2)+4 which are formed by direct ionization processes. Using the relative Franck–Condon factors for the O2 ã4Ru←X̃3S−g transitions, the relative reaction probabilities for the ion–molecule half reactions O+2(ã2Ru, v)⋅(O2)m→O+2m+1+O(m = 1, 2, and 3)—(1)—as a function of the vibrational quantum number v have been determined. The relative intensities of O+3, O+5, O+7, and (O2)+m = 2–4 observed in this experiment support the conclusion that the reaction probabilities of (1) with m = 2 and 3 are substantially larger than those with m = 1.