Jahn-Teller and L-uncoupling effects on rotational energy levels of symmetric and spherical top molecules

The effects of a small Jahn-Teller effect and of Coriolis L-uncoupling forces on the rotational energy levels of a degenerate electronic state are considered. In general the two types of mechanism make similar contributions to the energy levels, and can only be separated by the use of additional information such as the isotopic variation of the molecular parameters. The theory of the rotational energies is formulated in a way that is applicable to both symmetric and spherical top molecules. The expressions derived for symmetric tops reproduce the results of previous workers. For E or F electronic states of spherical tops, the rotational parameters are equivalent to those employed for the fundamental levels of degenerate vibrations, but they must be interpreted in terms of the form of the appropriate double or triple potential energy surface. Parameters calculated for the 3d²F₂-3s²A₁ transition of the ND₄ molecule from the ab initio potential surface of Havriliak and King do not agree well with the experimental values of Herzberg and Hougen.