Fluorescence in the dissociative excitation of sulphur dioxide by electron impact

Electron-impact excitation of sulphur dioxide at energies > ca. 10 eV results in optical emission from the à ³? and B~ ³S states of the SO radical. The threshold energy for production of the SO(à ³?) was found to be 10.6 ± 0.5 eV, a value similar to the minimum thermochemical energy requirements. Optical emission excitation functions for the SO(Ã) state show that in the energy region 10-20 eV an important excitation channel involves the triplet manifold of sulphur dioxide, although there is still a significant contribution from singlet excitation, particularly at higher energies. The SO(Ã) state is quenched by molecular SO₂ with near-unit collisional efficiency, with an estimated rate constant of 8.4 × 10^-11 cm³ molecule^-1 s^-1. Detailed studies on production of SO(B~) could not be undertaken owing to considerable spectral interference from molecular sulphur dioxide emission.