Highly degenerate coupled electronic states

To the best of our knowledge, most of the general theories of nonadia-batic transition has been formulated to treat one-dimensional nonadiabatic systems of an isolated avoided crossing, the Landau Zener theory being an example. On the other hand, the theory of PSANB (and SET as well as its approximation) is not limited to those specific cases. Nevertheless, we need to examine how practically PSANB works by comparing its results with the full quantum calculations in a couple of one-dimensional 

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Fig. 18.1 A dressed-state model that is used in the text to describe absorption, emission, and elastic (Rayleigh) and inelastic (Raman) light scattering. g) and. v> represent particular vibronic levels associated with the lower (1) and upper (2) electronic states, respectively. These are levels associated with the nuclear potential surfaces of electronic states 1 and 2 (schematically represented hy the parabolas). Rj are radiative continua— 1 -photon-dressed vibronic levels of the lower electronic states. The quasi-continuum L represents a nonradiative channel—the high-energy regime of the vibronic manifold of electronic state 1. Note that the molecular dipole operator /t couples ground (g) and excited (s) molecular states, but the ensuing process occurs between quasi-degenerate dressed states g,k and 5,0).

The JT theorem [40] states that if in the high-symmetry configuration of a nonlinear polyatomic system the electronic state F is degenerate, there is at least one low-symmetry coordinate in the linear terms of the vibronic coupling that... 

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