Fine structure and centrifugal distortion in the electronic and microwave spectra of O2and SO

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Abstract
The fine structure theory of 3Σ states in diatomic molecules is re-examined with particular emphasis on centrifugal distortion in the triplet splitting parameters. The theoretical results are compared with optical and microwave data for the 3Σ ground states of O2 and SO. Excellent agreement is found between theoretical predictions and experimental results concerning the centrifugal distortion in the spin-spin splitting for both O2 and SO. However, in contrast to ab initio calculations this agreement indicates that the second-order contribution to the spin-spin splitting is of minor importance. The theoretical value of the centrifugal distortion in the spin-rotation coupling constant is about twice the experimental one for O2. It is pointed out that a slight transition towards coupling case (c) yields a reasonable explanation for this discrepancy.