Fluorescence of indirectly excited low vibrational levels of the¹A₂ state of sulfur dioxide

Vibronic levels of the ¹A₂ state of SO₂ were excited by crossing a supersonic jet of SO₂ with laser radiation corresponding to cold bands in the 3045-3005 Å region, and the emission spectrum was detected under different laser-jet geometrical alignments. When the exciting laser beam was allowed to cross the jet just outside the spectrometer's field of view, a vibrational fluorescence spectrum of simultaneously excited low ¹A₂ levels was measured and was found to be independent of the exciting wavelength. This indicated that these levels were populated through inelastic collisions within the jet and that the low-lying ¹A₂ levels do in general fluoresce back to the ground state. Fourteen band origins near the ¹A₂(0,0,0)′ → ¹A₁(0,0,0)″ transition at 3581 Å were thus identified to within ±1 Å . An excitation spectrum recorded while monitoring the fluorescence of these collision-populated low ¹A₂ levels showed many cold bands that have not been detected before and also showed that the efficiency of energy transfer due to collisions depended on the initially excited vibronic level.