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

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.