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Key Words nonadiabatic dynamics, vibronic coupling, photochemistry, diabatic states, ultrafast processes
Abstract 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.
1. INTRODUCTION
The Born-Oppenheimer Approximation (1, 2) is the keystone to the visualization of chemical processes. By separating the electronic and nuclear motion, it enables us to picture molecules as a set of nuclei moving over a potential energy surface provided by the electrons. Whereas the validity of this approximation for the vast majority of chemistry is not in doubt, it is now clear that in many important cases the approximation breaks down. The nuclear and electronic motion then couple, and unexpected phenomena may arise.
This breakdown is particularly common in the photochemistry of polyatomic molecules, where there are a large number of energetically close-lying electronic states and many nuclear degrees of freedom. A particularly striking and important example of the result of the coupling between nuclei and electrons, termed vibronic coupling, is a conical intersection between electronic states. Conical intersections, also called photochemical funnels, provide pathways for ultrafast interstate crossing, i.e., on the femtosecond timescale. As a number of recent publications show, the existence and relevance of such intersections is no longer in doubt (3-7).
Early studies on conical intersections focused on the Jahn-Teller effect (8-10). The high symmetry of this problem makes it particularly amenable to simple models. The first demonstration of the effect of a conical intersection on a general, non-Jahn-Teller, system was made in the study of an unexpected band in the photoelectron spectrum of butatriene (11, 12). This band can only be explained by a breakdown of the...