Unexpectedly rich vibronic structure in supersonic jet spectra of sulfur dioxide between 360 and 308 nm

An extensive vibrational resolution study of SO₂ between 27780 and 32500 cm^-1, spanning absorption to the ³B₁, ¹A₂, and ¹B₁ electronic states, has been carried out via fluorescence excitation in a molecular beam, achieving substantial improvements in sensitivity and rotational cooling over previous studies. Characterization of the dependence of transition intensities on molecular beam conditions has been used to identify and separate the contributions of cold and hot transitions in the spectra. Vibrational-mode-selective, carrier-gas-dependent cooling is observed and is instrumental in the identification of a group of vibrational levels exhibiting novel Franck-Condon patterns and little or no cold absorption. At least 135 vibronic levels are identified in the energy range studied, adding 71 to the previous total reported from our own and other laboratories. Eight levels below 29000 cm^-1 are assigned to the ³B₁ electronic state, supported by a set of anharmonic constants to approximate the observed vibrational structure. In the higher energy region, the accepted two-singlet-electronic-state model is compared to the experimental results and shown to be inadequate to account for the large number of observed levels.