What are the basic mechanisms of electronic transitions in molecular dynamic processes?

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Abstract
The basic mechanisms of electronic transitions in molecular processes and their theoretical treatments are summarized and reviewed. These are the non-adiabatic (either radially or rotationally induced) transitions and the decay (auto-ionization) mechanisms of ‘superexcited states’. The interdisciplinarity of the concept of non-adiabatic transition is emphasized, and the present status of the semiclassical theory is inclusively summarized together with some numerical applications. Particular emphasis is put on the non-adiabatic tunnelling process which is supposed to be an important key mechanism for state (or phase) change in various fields. Definitions of two kinds of superexcited state are given, and their peculiarities and richness in their participating dynamic processes are explained. The multichannel quantum defect theory is outlined and recommended as a powerful theoretical tool for dealing with the various dynamic processes such as photo-ionization, photodissociation, autoionization, dissociative recombination and associative ionization. Some numerical applications are also presented in order to promote the understanding of the mechanisms. The underlying philosophy throughout this paper is to try to clarify the basic mechanisms of electronic transitions and to formulate them in a unified way as much as possible.