Effective spin-spin interactions and singlet-triplet transition intensities in a class of open-shell diatomic molecules

Ab initio calculations of the effective spin-spin parameter λ, and of electric dipole intensities for the forbidden ¹Σ⁺-³Σ^- transition, are reported for NH, NF, PH, PF, NCl and SO, and values of λ alone are calculated for O₂ and S₂. In general the values of λ agree to within 10 per cent with the experimental values, and the calculations enable the apparently irregular variation of λ within this class of molecules to be explained. The effect on λ of several approximations has been investigated, and it is found that the simplest model considered gives reasonable agreement with experiment, as others have found in comparable cases. This model employs single configuration wavefunctions in which the ³Σ^- orbitals are used to represent the isoconfigurational ¹Σ⁺ state; only the interaction with this one ¹Σ⁺ state is included, and only one-centre spin-orbit integrals are retained. Values of λ were calculated at three internuclear separations bracketing the experimental equilibrium separation, so that the vibrational and rotational dependence of λ could be studied. The agreement of the calculated transition intensities with experiment is surprisingly good. Although the level of approximation of the calculation makes it necessary to treat these theoretical transition moments with caution, they suggest that the ¹Σ⁺-³Σ^- transition in PH, hitherto unobserved, should be at least as strong as the corresponding known transition in NH.