Rotational Spectrum of SO in the Terahertz Region

The pure rotational spectrum of sulfur monoxide, SO, has been recorded in the X3Σ electronic ground state and a1Δ excited electronic state with the Cologne terahertz spectrometer in conjunction with a newly designed dc-discharge cell, covering the frequency range between 880 GHz and 1.05 THz. The expanded spectral coverage allowed us to observe ground state and vibrationally excited rotational transitions as high as NJ = 2526-2425 for SO and the isotopomers 34S16O, 33S16O, and 32S18O in natural abundance. The vibrational and isotopic dependence of various molecular parameters such as the mass-independent rotational constants Uij, the spin-spin coupling constants Lij, and the spin-rotation constants Gij have been determined. Although these values are in very close agreement with earlier parameters of Bogey et al. [Chem Phys. 66, 99-104 (1982)], it was necessary to include higher order terms in order to achieve a satisfactory fit to the combined data set, consisting of all previous and newly acquired submillimeter-wave lines. On the basis of these data, the rotational spectrum of SO can now be predicted up to about 1.5 THz with a high level of accuracy, which will be important for future astrophysical identifications.