Franck-Condon principle and large change of shape in polyatomic molecules

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Abstract
The quantitative application of the Franck-Condon principle to polyatomic molecules has usually been restricted to electronic transitions in which only a small change of dimensions occurs. We have applied the Franck-Condon principle to transitions in which there are large changes of dimensions. In some cases a change of symmetry also occurs. In most of the examples considered a satisfactory correlation between the theoretical and the experimental results was obtained. If in an electronic transition the final state is less symmetrical than the initial state it is possible to decided whether the transition correlates with an allowed or a forbidden transition in the more symmetrical configuration. A recursion method is described which aids the calculation of intensity contours up to large quantum numbers.