Hund's coupling case (c) in diatomic molecules. I. Theory

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Abstract
General formulae are derived for the rotational energy and fine structure of Hund's case (c) electronic states in diatomic molecules. The calculation is based on a perturbation approach in which the total electron Hamiltonian (including relativistic effects) is taken as the unperturbed Hamiltonian. The'small' Coriolis term -2BJJa (which lead to a case transition) then represents the only perturbation operator of interest. This enables the calculation of converge rapidly, with the consequent result that one obtains compact intermediate case (c)-case (e) energy formulae of high accuracy. Furthermore it is shown how the standard case (a)-case (b) formulae emerge as special cases of the general intermediate case (c)-case (e) expressions. The present treatment is confined to the following values of mod Omega mod : mod Omega mod =1/2, 3/2, 0, 1 and 2, which are believed to represent the states observable in practice.