The first ionization potential of O2 has been found to correspond to 1027.8±0.1 Å (12.063±0.001 eV). Previous determinations of the ionization potential are reviewed and compared with the present result. The photoionization cross section measured from threshold to 993 Å reveals vibrational structure superimposed on autoionized absorption peaks. The absorption spectrum illustrates the profuse absorption bands in the vicinity of the ionization threshold. Ionization was observed at wavelengths longer than 1027.8 Å and is explained on the basis of the formation of excited O2. From the interpretation of the results, the dissociation energy for the ground state of the ion is 6.669±0.001 eV and the electron affinity of O2 is ≥0.21 eV.
JAMES A. R. SAMSON and R. B. CAIRNS, "Ionization Potential of O2," J. Opt. Soc. Am. 56, 769-774 (1966)