Theory of the Condon Locus

Abstract

The Condon locus for a diatomic molecule is the locus, in the plane, of the strongest bands in an electronic band system. The form of the locus depends upon the form of the potential energy function of the electronic states involved. We show how the locus depends on the potential energy function for simple harmonic and anharmonic oscillators, first from a classical point of view, and then from a quantum mechanical point of view. One phenomenon of interest is that, in the case of anharmonic oscillators, the upper branch of the Condon locus traces much stronger bands than the lower branch. Another phenomenon, predicted by quantum mechanics but not by classical mechanics, is the existence of secondary nested Condon loci.