Non-Adiabatic Interactions in the Unimolecular Decay of Polyatomic Molecules
It is well-known that intersections, or avoided intersections between potential energy hypersurfaces play an important role in the interpretation of photochemical processes (1). In such regions of space, the Born-Oppenheimer approximation breaks down and the behavior of the system is no longer determined by the usual potential energy surfaces (in the Born-Oppenheimer sense), i.e. by the eigenvalues of the electronic hamiltonian. A coupling between (or among) energy surfaces takes place. Several methods are available to calculate the transition probabilities between them. Since the case of polyatomic molecules is fairly complicated, we shall restrict ourselves to the simplest method, called the semi-classical approximation (2) : the nuclear motion is described by a classical trajectory, whereas the electronic motion is treated quantum-mechanically.
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