Nonadiabatic effects play an important role in many areas of physics and chemistry. The coupling between electrons and nuclei may, for example, lead to the formation of a conical intersection between potential energy surfaces, which provides an efficient pathway for radiationless decay between electronic states. At such intersections the Born-Oppenheimer approximation breaks down, and unexpected dynamical processes result, which can be observed spectroscopically. We review the basic theory required to understand and describe conical, and related, intersections. A simple model is presented, which can be used to classify the different types of intersections known. An example is also given using wavepacket dynamics simulations to demonstrate the prototypical features of how a molecular system passes through a conical intersection.
Penning ionization electron spectroscopy of C 6 H 6 by collision with He * (2 3 S) metastable atoms and classical trajectory calculations: Optimization of ab initio model potentials J. Chem. Phys. 122, 044303 (2005); 10.1063/1.1834900Doppler-free two-photon excitation spectroscopy and the Zeeman effect of the 14 0 1 band of the S 1 1 B 2u S 0 1 A 1g transition of benzene-d 6
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