The Franck-Condon principle for polyatomic molecules

The applicability of an r-centroid approximation to polyatomic molecules is discussed. The transformation between sets of normal coordinates for the same distorted molecule, but related to different reference configurations is investigated and shown to be a function of the distortion. The significance of this fact for the interpretation of Franck-Condon factors in polyatomic molecules is examined in relation to other approximations currently made.