Knowledge Bank

The technique of cavity ring-down (CRD) spectroscopy is particularly useful for measuring absorptions of very weak optical transitions. We have in this manner investigated the $40950-41300cm-1$ region in $O2$, where only absorption in the $O2(A3\Sigma u+-X3\Sigma g-$) 11-0 band had been previously identified. Five new bands have been discovered in this range - the ($A3\Delta u-X3\Sigma g-$) 12-0 and 13-0 bands, the $c1\Sigma u--X3\Sigma g-$ 17-0 and 18.0 bands, and the $A3\Sigma u+-X3\Sigma g-$ 12-0 band. The origins of the $F1$ and $F2$ components of the latter lie only $7cm-1$ below the lowest dissociation limit, and 14 lines have been identified. No $F3$ levels were observed; apparently all are above the dissociation limit. The high instrumental sensitivity of the CRD technique has allowed observation of weak lines of the A-X 11-0 band, and 12 of the 13 branches have been identified and their intensities measured. A very low upper limit has been set on the intensity of the thirteenth branch $Q13$. We find 106 unidentified lines in the region, the stronger ones (18) lying in the vicinity of lines of the A-X 11-0 band. The weaker ones (88) are spread throughout the spectral region, up to and even beyond the $O2$ dissociation limit, and probably have their origin in transitions to very weakly bound $O2$ states, which may have atmospheric significance. These weaker lines have intensities that are typically 1-5% of the strong A-X 11-0 band lines.