Rotational Structure at the Long Wavelength End of the 2900 Å System of SO2

Rotational analyses have been carried out for the three longest wavelength bands observable in the '2900 Å system' of SO2 (at 3340, 3359, and 3395 Å). Although the bands are perturbed quite severely it has been possible to prove that they are type C bands, and that the approximate upper state geometric structure of the molecule is r(S—O) = 1.53 Å, [Formula: see text]. Comparison with the theoretical calculations of Hillier and Saunders now leaves little doubt that the upper electronic state is the π → π* 1A2 state, appearing in absorption by Herzberg–Teller mixing through Q3′(b2), as recently proposed by Dixon and Hallé. A vibrational numbering for these bands is given, which is consistent with the inertial defect obtained from the rotational analysis, and with recent isotope data published by Brand and Nanes: the 3395 Å band appears to be 031–000, and the electronic origin falls near 27 930 cm−1.