FranckCondonÜberlappungen

The Franck-Condon principle is a fundamental concept in molecular spectroscopy and physical chemistry that describes the vertical transition of an electron between vibrational energy levels of two electronic states of a diatomic or polyatomic molecule. Named after physicist John William Nicholson Franck and physicist Gustav Herzberg Condon, the principle was first proposed in the 1920s to explain the observed intensity distribution of vibrational bands in electronic spectra.

At its core, the Franck-Condon principle states that electronic transitions occur so rapidly compared to nuclear motion that nuclei do not have time to adjust their positions during the transition. This results in transitions occurring "vertically" on a potential energy diagram, meaning the nuclei remain in their initial vibrational state. Consequently, the intensity of vibrational bands in an electronic spectrum is determined by the overlap (Franck-Condon factors) between the vibrational wavefunctions of the initial and final states.

The principle is particularly useful in interpreting the vibrational structure of electronic spectra, such as those observed in fluorescence, phosphorescence, and absorption. For instance, in the case of a diatomic molecule undergoing an electronic transition from a ground state to an excited state, the intensity of each vibrational band corresponds to the probability of the transition occurring while the nuclei remain in the same vibrational level. This vertical transition leads to a characteristic "stick spectrum," where individual vibrational lines are prominent, rather than a smooth, broadened continuum.

Mathematically, the Franck-Condon factors are calculated using the overlap integral of the vibrational wavefunctions of the initial and final states, which depends on the Franck-Condon factors of the potential energy curves. The shape of these curves—whether they are similar, inverted, or significantly displaced—determines the intensity distribution of the observed spectral lines. The principle also extends to polyatomic molecules, where it helps explain the complex vibrational structure of their electronic spectra.