Fine structure of3Π and3Σ-states in diatomic molecules

The fine structure of the A 3 Pi and X 3 Sigma - states of ND, NH and OH + has been investigated. An iteration procedure based on numerical matrix diagonalization takes the 3 Pi - 3 Sigma - interaction completely into account, and contributions from singlet states are considered to second order. Centrifugal distortion terms are included in the off-diagonal 3 Pi - 3 Sigma matrix elements through a quantum mechanical approach, and these terms are found to yield the correct explanation for the Lambda doubling anomalies encountered for high rotational quantum numbers. The dependence of the coupling constant A on the rotational quantum number observed by previous investigators is removed, even for the highest rotational quantum numbers (J=30), through the inclusion of higher order terms and the spin-rotation interaction. The A 3 Pi state of AsD and AsH is briefly commented upon, and the relation A=A 0 +2A J J(J+1) is found to explain certain anomalies in the multiplet splitting. For the 3 Sigma - state the unified treatment of the 3 Pi - 3 Sigma - interaction leads to some further conclusions regarding the origin of the splitting.