Abstract Scientific and educational literature often describes vibronic transition probabilities of molecules within the framework of Franck–Condon’s (FC) “principle” and “factors.” The “principle” refers to the verticality of the optical electronic transition at a particular point of the molecular geometry, while the “factors” describe the transition probabilities between specific vibrational levels of two combining electronic states. At first glance, though they describe the same process, these two concepts seem to be in contradiction with each other, especially with regards to the question of where, exactly, the point at which this vertical transition occurs is. This paper discusses the origin of this apparent contradiction and ways to resolve it. Semi-classical and quantum models of bound-bound and bound-continuum vibronic transitions in diatomic molecules are considered. It is shown that, for both models, it is possible to calculate the vertical transition points, and the conditions are determined, under which the results obtained using both models coincide.
Stolyarov et al. (Sun,) studied this question.