Vibronic Interactions and the Jahn-Teller Theorem

Abstract

In this chapter the first principles which form the basis of the concept of vibronic interactions, the concept of mixing electronic states by nuclear displacements, are discussed. After analysis of the traditional adiabatic approximation and its limitations (Sec. 2.1), the so-called vibronic Hamiltonian which takes into account the nonadiabatic interaction of electrons and nuclei is deduced (Sect. 2.2). The proof and discussion of the Jahn-Teller theorem are presented in Sect. 2.3 by considering the shape of the adiabatic potential surface in the neighborhood of the point of electronic degeneracy where the deviations from the adiabatic approximation are most marked. This theorem served as a starting point for the development of the field as a whole, and therefore the physical consequences of the nonadiabatic vibronic mixing of electronic states are often called the Jahn-Teller effects.