Knowledge Bank

We have cooled a dilute mixture of $SO2$ in helium by expansion through a supersonic nozzle and have studied the fluorescence excitation spectrum excited by a pulsed tunable dye laser. The supersonic expansion produced a rotational temperature of less than 1 K. The dye laser was pumped by the second harmonic of a Nd:YAG laser, and used an oscillator cavity containing a glancing incidence grating. This oscillator produced tunable pulsed visible light of $\sim 5$ GHz spectral bandwidth, and this was amplified in three amplifier stages and frequency doubled into the ultra-violet by a type 1 phase matched KDP crystal. The $SO2$ spectrum was examined in the region from 3000 to 3300{\AA}. The spectrum in this region was complex with over 50 discemible bands, but it was sparse relative to the spectrum of the static gas in this region. The individual rotational lines of a number of bands were well fit by the rotational constants given by Hamada and $Merer1$ for the band at 3395{\AA}. We will discuss the spectrum in terms of a $model2$ in which a $\mathrm{<mrow\; data-mjx-texclass="ORD">1</mrow>}Bl$ excited state is both Herzberg-Teller coupled to a lower lying $\mathrm{<mrow\; data-mjx-texclass="ORD">1</mrow>}A2$ electronic state and Renner-Teller couples to the ground $X~1A1$ electronic state.