| Vibronic Coupling |
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| quantum chemistry | |
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Vibronic coupling is large in the case two adiabatic Potential Energy Surface s come close to each other, i.e. when the energy gap between them is of the order of magnitude of one oscillation quantum. This usually happens in the neighbourhood of an Avoided Crossing of Potential Energy Surface s corresponding to distinct electronic states of the same spatial and spin symmetry. However, this is not the only situation in which vibronic coupling exists. In this case the Adiabatic or Born-Oppenheimer Approximation fails and non adiabatic terms (the so-called vibronic coupling terms) have to be taken into account. The vibronic coupling terms are usually difficult to evaluate. This is due to the fact that they are proportional to the first and second derivatives of the electronic wave function with respect to the Molecular Coordinates . A simpler way to solve this problem is to switch from the Adiabatic to the Diabatic representation of the Potential Energy Surface s. The vibronic terms are responsible for example for surface hopping or the Berry Phase . The Berry Phase has been discovered by Longuet-Higgins in this context. The vibronic coupling becomes infinite in the neighbourhood of a Conical Intersection . This singularity in the potential energy landscape is the origin of the Berry Phase . Perhaps the earliest demonstration of the importance of vibronic coupling was during the 1930's. Calculations of the lower Excited Levels of Benzene by Sklar in 1937 (with the valence bond method) and later in 1938 by Goeppert-Mayer and Sklar (with the Molecular Orbital method) demonstrated a correspondence between the theoretical predictions and experimental results of the benzene Spectrum . The benzene spectrum was the first qualitative computation of the efficiencies of various vibrations at inducing intensity absorption. {Link without Title} REFERENCES 1) Fischer, Gad. "Vibronic Coupling - The Interaction between the Electronic and Nuclear Motions", Academic Press, New York, 1984. ISBN 0-12-257240-8 SEE ALSO |