The excitation of band systems by electron impact

The relative positions of the more intense bands of a system in the usual square array can be predicted by classical methods (the Frank-Condon principle) from the potential curves for the initial and final states. This procedure is upheld in its essentials by wave mechanics, and a further wave mechanics development permits the calculation of transition probabilities in emission in band systems of symmetric diatomic molecules. The essential features of the theory are that relatively massive nuclei may be assumed not to respond immediately to changes in the electronic arrangement of the molecule, and that one need not evaluate that part of the electric moment directly concerned with such electronic rearrangements. These circumstances reduce the problem to one for simple oscillators with known potential functions. For a complete theory of the relative intensities, however, the details of the accompanying phenomenon of excitation must be known. While these are necessarily extremely complicated for bands emitted from different initial electronic states, a considerable simplification may be expected for bands emitted from the same electronic state. The problem of the relative excitation probabilities then forms the excitation analogue of the Frank-Condon principle.