Vibrational structure in the low energy region of the ¹A₂ state of SO₂

Molecular beam fluorescence excitation spectra of sulphur dioxide in the wavelength range 348-334 nm are presented. Identification of cold transitions is accomplished by controlled variation of vibrational temperature, and narrow rotational contours permit accurate determination of the corresponding vibrational band origins. These data conform well with the proposed assignment of ¹A₂-¹A₁ vibrational progressions of Hamada and Merer, although different progressions are found to display markedly different intervals. The energy below which bands of the ³B₁-¹A₁ transition exceed those of the ¹A₂-¹A₁ system in intensity is found near 347 nm.