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Published June 1, 1970 | public
Journal Article Open

Is the Breakdown of the Born-Oppenheimer Approximation Responsible for Internal Conversion in Large Molecules?


Vibronic radiationless transitions in large polyatomic molecules can be thought of as a process whereby the molecule, initially prepared in a discrete quasistationary state, makes a transition to an adjoining vibronic continuum belonging to a lower electronic state of the same multiplicity. In many instances the transition is analogous to penetration through a barrier between two "nested" potential sheets far away from an actual intersection of the sheets. Simultaneous distortions of vibrational and electronic parts of the wavefunction are required for such a tunneling process. The vibrational distortion manifests itself in the familiar Franck-Condon effect. The electronic distortion can be caused by nontotally symmetric vibrations of the molecule either because of the displaced nuclear configuration, to which the electrons instantly respond (Herzberg-Teller effect), or because of the protracted response of the electrons to the kinetic energy of nuclear motion (nonadiabatic effect or the breakdown of the Born-Oppenheimer approximation). It is found here that Herzberg-Teller coupling is more effective in causing radiationless transitions than the breakdown of the Born-Oppenheimer approximation when the potential surfaces involved are not close to an intersection.

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Copyright © 1970 by the National Academy of Sciences. Communicated by George S. Hammond, October 27, 1969. This work was supported in part by the U.S. Army Research Office, Durham, N.C., under contract DA-31-124-ARO-D-370. Arthur Amos Noyes Laboratory of Chemical Physics, Contribution No. 3865.


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