Franck-Condon principle The Franck-Condon principle is a rule in spectroscopy that explains the intensity of vibronic transitions. Vibronic transitions are the simultaneous changes in electronic and vibrational energy levels of a molecule due to absorption or emission of a photon of appropriate energy. Electronic transitions are essentially instantaneous, so there is no time for appreciable motion of the nuclei. So the transitions appear as vertical lines with no change in internuclear distance. This is referred to as Franck-Condon principle. The spectra are strongly affected by the probability that an electron is at a location to contribute to such a "vertical" transition. That probability is the wave function squared, and at least the lowest vibrational states are approximated by the quantum harmonic oscillator.