THE $A^{3}\Sigma_{u}^{+}, A'\ ^{3}\Delta_{u}$, AND $c^{1}\Sigma_{u}^{-}$ STATES OF $O_{2}$ IN THE LABORATORY AND IN PLANETARY AIRGLOWS

Abstract

Until recently, studies of the three $O_{2}$ metastable states lying 4 to 5 eV above the ground state have been of a quite limited nature. The $A^{3}\Sigma_{u}^{+} \rightarrow X^{3}\Sigma_{g}^{-}$ transition is known both in laboratory afterglow sources and in the terrestrial nightglow, but investigation of the $c^{1}\Sigma_{u}^{-}$ and $A^{\prime}\ ^{3}\Delta_{u}$ states has been nearly dormant since Herzberg’s absorption measurements of 27 years ago. It is now clear that all three states are produced by O-atom recombination. Under proper afterglow conditions, the $c^{1}\Sigma_{u}^{-} \rightarrow X^{3}\Sigma_{g}^{-}, c^{1}\Sigma_{u}^{-} \rightarrow a^{1}\Delta_{g}$, $A^{\prime} {^{3}\Delta_{u}^{-}} \rightarrow X^{3}\Sigma_{g}^{-}$, and $A^{\prime} {^{3}\Delta_{u}} \rightarrow a^{1}\Delta_{g}$ emission band systems are all easily observed, providing valuable spectroscopic information. The c-X bands are prominent in the Venus nightglow spectrum. We have now confirmed that the $A^{\prime}-a$ bands are distinct terrestrial nightglow features. By calculating a synthetic emission spectrum we are able to match the observed 3000-4500 {\AA} terrestrial nightglow spectrum only if we include bands from the c-X system, in addition to the previously identified A-X and $A^{\prime}-a$ bands. All three upper states emit exclusively from high vibrational levels. In contrast the $c^{1}\Sigma_{u}^{-}$ state in the Venus atmosphere is seen only from $v^{\prime}=0$. The emissions from $A^{\prime}$ and c states account for 25-30%. of the 2500-4500 {\AA} terrestrial nightglow, implying that these states are much more important than was previously suspected.