Experimental observation of the lowestΣu+1valence state ofO2

Transitions into the lowest ${}_{}{}^1{}_{}{}^{}\mathrm{\Sigma }_{\mathit{u}}^{+}{}_{}{}^{}$ valence state of ${\mathrm{O}}_2$, which we shall name f′${\mathrm{}}^1$ ${\mathrm{\Sigma }}_{\mathit{u}}^{+}$, are observed. A total of nine weak absorption bands is found from the system f′${\mathrm{}}^1$ ${\mathrm{\Sigma }}_{\mathit{u}}^{+}$←X${\mathrm{}}^3$ ${\mathrm{\Sigma }}_{\mathit{g}}^{\mathrm{-}}$ for the isotopes ${}_{}{}^{16}{}_{}{}^{}\mathrm{O}_2^{}{}_{}{}^{}$ and ${}_{}{}^{18}{}_{}{}^{}\mathrm{O}_2^{}{}_{}{}^{}$. The observed band origins and rotational constants are found to be significantly perturbed due to an electrostatic interaction between the valence state f′${\mathrm{}}^1$ ${\mathrm{\Sigma }}_{\mathit{u}}^{+}$ and the Rydberg states 3p${\mathrm{\pi }}_{\mathit{u}}$ f${\mathrm{}}^1$ ${\mathrm{\Sigma }}_{\mathit{u}}^{+}$ and 4p${\mathrm{\pi }}_{\mathit{u}}$ j${\mathrm{}}^1$ ${\mathrm{\Sigma }}_{\mathit{u}}^{+}$. In addition, rotational perturbations observed in certain vibrational levels of the Rydberg states f${\mathrm{}}^1$ ${\mathrm{\Sigma }}_{\mathit{u}}^{+}$ and j${\mathrm{}}^1$ ${\mathrm{\Sigma }}_{\mathit{u}}^{+}$ are found to be caused by the f′${\mathrm{}}^1$ ${\mathrm{\Sigma }}_{\mathit{u}}^{+}$ state through the same valence-Rydberg interaction. The f′←X bands are found to be predissociating resonances that exhibit Beutler-Fano line shapes of widely varying asymmetry. In particular, the (13,0) band of