Theoretical and Experimental (Electron-Impact) Studies of the Low-Lying Rydberg States inO2

Theoretical studies of the $n=3,4,\mathrm{and} 5$ Rydberg series in ${\mathrm{O}}_2$, converging to ${\mathrm{O}}_2^{+}\left(X {}_{}{}^2{}_{}{}^{}\Pi _g^{}{}_{}{}^{}\right)$, are presented and correlated with electron energy-loss spectra of molecular oxygen, in the region from 7.0 to 10.5 eV. The energy-loss spectra have been studied as a function of the scattering angle for a incident electron energy of 45 eV. The angular dependence determined for various features in the energy-loss spectra have been correlated with the known behavior for transitions between electronic states of certain symmetry, and with the theoretical results, to identify the low-lying Rydberg states. Four vibrational levels of the $\left(3s {\sigma }_g\right) {}_{}{}^3{}_{}{}^{}\Pi _g^{}{}_{}{}^{}$ Rydberg state have been observed superposed on top of the maximum intensity portion of the Schumann-Range continuum with $T_{00}=8.145\mathrm{}\pm 0.020$ eV. The theoretical results, combined with the energy-loss measurements and the high-resolution photon-absorption work, lead to the assignment of the lowest dipole-allowed ${}_{}{}^3{}_{}{}^{}\Sigma _u^{-}{}_{}{}^{}$ and ${}_{}{}^3{}_{}{}^{}\Pi _u^{}{}_{}{}^{}$ Rydberg states at $T_{00}\le 9.31$ eV and $T_{00}=9.97\mathrm{}$ eV, respectively.