High resolution threshold photoelectron spectrum of oxygen in the 12–19 eV region

Abstract

The threshold photoelectron spectrum of oxygen has been measured with high resolution in the photon energy range of 12–19 eV using a penetrating field technique and a synchrotron radiation source. The long vibrational progression of the X2Πg state of O2+ was observed to v′=45. An extrapolation in a Birge–Sponer plot (ΔG versus v′+1/2) gives a maximum vibrational level vmax′=56. The A2Πu state was also measured in detail. Most vibrational levels of the A2Πu state of O2+ (from v′=0 to 24) were observed for the first time. A Birge–Sponer plot of the A2Πu state gives a maximum vibrational level vmax′=31. The dissociation limit of the A2Πu state is slightly higher (76 meV) than the well-known dissociation limit O(3P2)+O+(3S0) 18.733 eV. Therefore the A2Πu state may be considered to have a hump. We observed new states which were assigned to Σg+4 and Σu+2 according to a previous theoretical investigation [Beebe et al., J. Chem. Phys. 64, 2080 (1976)]. Potential curves of the X2Πg and A2Πu states were calculated by the Rydberg–Klein–Rees method using both the rotational constants obtained previously by others [K. P. Huber and G. Herzberg, Molecular Spectra and Molecular Structure, Constants of Diatomic Molecules Volume IV (Van Nostrand, New York, 1974)] and the vibrational energy values observed in the present experiment. A Le Roy–Bernstein plot as well as a Birge–Sponer plot were used to determine the dissociation energy of the X2Πg state. However, the plot gave an unreasonably large dissociation limit.