The Franck-Condon Principle describes the intensities of vibronic transitions, or the absorption or emission of a photon. It states that when a molecule is undergoing an electronic transition, such as ionization, the nuclear configuration of the molecule experiences no significant change. This is due in fact that nuclei are much more massive than electrons and the electronic transition takes place faster than the nuclei can respond. When the nucleus realigns itself with with the new electronic configuration, the theory states that it must undergo a vibration.

The nucleus in a molecule has Coulombic forces acting on it from the electrons and other nuclei of the system. Once a molecule undergoes the electronic transition, the resulting Coulombic forces serve to change the energy of the molecule. This change brings it from the ground state to an excited state and results in the nuclei changing its vibrational state. This vibrational structure of an electronic transition shows that the absorption spectrum consists of many lines instead of a single sharp electronic absorption line.

Figure removed due to potential copyright violations

In this figure, the nuclear axis shows a consequence of the internuclear separation and the vibronic transition is indicated by the vertical arrows. This fugure demonstrates three things: An absorption leads to a higher energy state, fluorescence leads to a lower energy state, and the shift in nuclear coordinates between the ground and excited state is indicative of a new equilibrium position for nuclear interaction potenial. The fact that the fluorescence arrow is shorter than the absorption indicates that it has less energy, or that its wavelength is longer.

This change in vibration is maintained during a state termed the rapid electronic excitation. The resulting Coulombic forces produce an equilibrium as shown in the figure for the nuclei termed a turning point. The turning point can be mapped by drawing a vertical line from the minimum of the lower curve to the intersection of the higher electronic state. This procedure is termed a vertical transition.

The IUPAC definition is as follows: The Franck-Condon Principle has both a Classical and Quantum application. Classically, the Franck–Condon principle is the approximation that an electronic transition is most likely to occur without changes in the positions of the nuclei in the molecular entity and its environment. The resulting state is called a Franck–Condon state, and the transition involved, a vertical transition. The quantum mechanical formulation of this principle is that the intensity of a vibronic transition is proportional to the square of the overlap integral between the vibrational wavefunctions of the two states that are involved in the transition.

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