CHARACTERIZATION OF THE $\tilde{b}^{3}A_{2}$ STATE OF $SO_{2}$

Abstract

Every vibrational level of the ${\tilde{a}}^{3}B_{1}$ state of $SO_{2}$ 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 $S^{16}O_{2}$ and $S^{18}O_{2}$ 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 ${\tilde{a}}^{3}B_{1}$ 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 $^{3}B_{1}$ perturbing levels. A plot of the energies of the perturbations against $K^{2}$ reveals the courses of the unseen perturbing levels as a series of nearly parallel straight lines connecting perturbations in different vibrational levels of the $\tilde{a}^{3}B_{1}$ 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 $A - \bar{B}$ and $\bar{B}_{1}$ determined for the perturbing state correspond closely to those of the $A^{1}A_{2}$ state, indicating that the perturbing levels are vibrational $b_{2}$ levels of the $b^{3}A_{2}$ state. The electronic matrix element $\langle{}^{3}A_{2} | {\partial} / {\partial} Q_{3} | ^{3}B_{1}\rangle$ can be estimated from the sizes of the various perturbations.