Knowledge Bank

Collisions of oxygen or nitrogen molecules with laser-excitated high vibrational levels of $O2$ produce a longer lived excited state, whose resonant multiphoton ionization (REMPI) spectrum cannot be assigned to any known singlet or triplet system [1]. We attribute the lower state of this new transition to the predicted [2,3] but previously unobserved $\mathrm{<mrow\; data-mjx-texclass="ORD">5</mrow>}\Pi g$ valence state of $O2$. The regular sequence of vibrational bands observed suggests that the upper state is an ion-pair state of $O2$, dissociating to $O++O-$. Two vibrational levels have been observed in the lower state and ten in the upper state, the latter in the range 97,000 to $100,000cm-1$. Ab initio calculations [2,3] indicate that, although quite weakly bound, at large internuclear distances the $\mathrm{<mrow\; data-mjx-texclass="ORD">5</mrow>}\Pi g$ valence state should be the lowest of the states dissociating to ground state atoms. Because of its high degeneracy it could be a key intermediate state in $O+O$ recombination [4,5]. We are developing a more detailed and quantitative theoretical model of $O+O+M$ collisions. In addition to the experimental spectroscopy and kinetics results, we will also present preliminary work on spin-orbit resolved long-range potential curves for $O2$ and implications for oxygen atom recombination and the role of $O2(5\Pi g)$ in predicting atmospheric nightglow emissions [6].