Knowledge Bank

Rotational analyses of various bands of the 2900 {\AA} absorption system of $SO2$ have been carried out, from high resolution plates taken with the gas at dry ice temperature. Fairly complete analyses of the three longest wavelength bands observable in the transition (3340 - 3400 {\AA}) show that the upper state structure is r(S - 0) = 1.53 {\AA}, OS0 = $90\circ$, and that the bands are type C; the transition must be interpreted as the forbidden $\pi \to \pi \ast 1A2-1A1$ transition, made allowed by Herzberg-Teller interaction through $Q3$ ($b2$). Partial analyses of some of the strong bands near 3000 {\AA} have also been performed. These bands are very similar to the long wavelength bands (chough they are even more severely perturbed), and, in accord with the recent $S18O2$ isotope shift data of Brand and Nanes$\mathrm{<mrow\; data-mjx-texclass="ORD">1</mrow>}$, must also belong to the $\mathrm{<mrow\; data-mjx-texclass="ORD">1</mrow>}A2-1A1$ transition. The intensity of these bands presumably comes from the $n\to \pi \ast 1B1(1\Delta g)-1A1$ transition; it seems that this transition falls near 3000 {\AA}.