A Class of Perturbations of Molecular Levels

By following Van Vleck and Kronig, the conditions for the occurrence of perturbations are investigated in detail for a simple case. Class A perturbations which are produced by the rotation of the molecule must have ${\Lambda }^{\prime }=\Lambda \pm 1$. The vibrational levels cannot be perturbed in this case. There will be a constant shift between the rotational levels before and after the perturbations. When the electronic motion can be described approximately by the precession of a constant angular momentum about the figure axis, the components of the perturbation matrix can be completely calculated. For class B perturbations $\Lambda ={\Lambda }^{\prime }$. The vibrational levels may be perturbed and there is no constant shift between the levels for high and low values of $J$.