A new, optically forbidden Rydberg series in O2 converging to the O+2 c 4Σ−u limit

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Abstract
We have measured the angular dependence of inelastically scattered electrons in O2 in the energy‐loss range 16 to 26 eV for incident electron energies between 75 and 400 eV, and for scattering angles between 2° and 12°. For high incident energy and low scattering angle our energy loss spectra correspond to the known optical absorption spectrum. At higher scattering angles, four new structures appear in our spectra at 21.85, 23.30, 23.80, and 24.06 eV. These four structures are the lowest members of the first optically forbidden Rydberg series observed to converge to the O+2 c 4Σ−u limit at 24.56 eV. The calculated quantum defects for these levels are about 0.75. This value, combined with angular scattering propensity rules, indicates that the new Rydberg states occur by promotion of an electron from the σu2s orbital to npσu orbitals, where n = 3,4,5,6. A consideration of the propensity rules for excitation of forbidden transitions by high energy electron impact suggest the term symbol of the new Rydberg states to be 3Σ−g.