Knowledge Bank

Every vibrational level of the $a~3B1$ state of $SO2$ except the 000 level has been found to suffer at least one local rotational perturbation; at higher energies these become extremely severe, and the vibrational structure become chaotic. By comparing the spectra of $S16O2$ and $S18O2$ we have shown that similar perturbations occur at approximately the same energies in the two isotopes, which can be explained in terms of a single perturbing electronic state, lying only a few hundred $cm-1$ above the $a~3B1$ state, From rotational analysis of some of the Less severe perturbations we have shown that they are mainly homogeneous ($\Delta K=0$), corresponding to vibronic $\mathrm{<mrow\; data-mjx-texclass="ORD">3</mrow>}B1$ perturbing levels. A plot of the energies of the perturbations against $K2$ reveals the courses of the unseen perturbing levels as a series of nearly parallel straight lines connecting perturbations in different vibrational levels of the $a~3B1$ state. It is found that the perturbing levels form a regular series, with separation $\sim 320$ $cm-1$ (representing the bending frequency of the perturbing state). The rotational constants $A-B¯$ and $B¯1$ determined for the perturbing state correspond closely to those of the $A1A2$ state, indicating that the perturbing levels are vibrational $b2$ levels of the $b3A2$ state. The electronic matrix element $⟨3A2|\partial /\partial Q3|3B1⟩$ can be estimated from the sizes of the various perturbations.