Accurate rotational spectroscopy of sulfur dioxide, SO2, in its ground vibrational and first excited bending states, v2Â =Â 0, 1, up to 2Â THz

Approximately 150 pure rotational transitions each have been recorded for SO2, v2 = 0 and 1, in selected frequency regions up to 2 THz. The J and Ka quantum numbers reach very high values: 92 and 23, respectively, for the ground vibrational state and 81 and 21, respectively, for the first excited bending state. The highest levels accessed are almost 3000 cm−1 above ground. The relative experimental uncertainties Δν/ν are about 10−8 for several medium to strong, isolated lines, and generally better than 2.5 × 10−7. Improved spectroscopic parameters have been obtained for both states, particularly for the excited bending state. In fact, the accuracies with which the energy levels of the v2 = 1 state are known depend essentially only on the accuracy with which the vibrational spacing is known from infrared spectroscopy.