Nonadiabatic coupling effects

If the Born-Oppenheimer approximation is not valid—for example, in the vicinity of surface crossings—nonadiabatic coupling effects (that couple nuclear and electronic motion) need to be taken in account to correctly describe the motion of the molecular system. This is done, for instance, when one needs to describe a jump between two different PESs. In this case, one uses semiclassi-cal theories and the surface-hopping method, which we discuss subsequently. We now discuss in some detail how the region in which nonadiabatic effects become important can be characterized topologically. 

These strong non-adiabatic effects observed in the cone-states of the upper sheet contrast with the absence of any significant effect in the H-I-H2 reactive collision. Eor instance, Mahapatra et al. [69] examined the role of these effects in the H -f H2 (v = 0, = 0) reaction probability for / = 0 and found negligible nonadiabatic coupling effects in the initial state selected probability. Subsequently, Mahapatra and co-workers [70] reported initial state-selected ICS and thermal rate constants of H -I- H2(HD) for total energies up to the three body dissociation. Again, they... 

Photon polarization-dependent nonadiabatic coupling effects of coherently excited quasi-degenerate electronic states are also explained by an analytical treatment. 

We explain these nonadiabatic coupling effects of coherently excited quasidegenerate electronic states in terms of a simplified one-dimensional model analysis. 

To proceed, we note that the initial state (0)) usually represents a coherent state on the excited-state potential surface (because of the assumed broadband excitation). Moreover, as emphasized in Sec. 2 above, the strong nonadiabatic coupling effects at conical intersections may lead to a pronounced mixing of vibrational levels of the upper and lower of the intersecting surfaces. For these reasons, it is appropriate to introduce the overall electronic population Pi t) (of electronic state i) as a direct and natural measure of the internal-conversion dynamics on strongly coupled surfaces. The population of electronic state 1 ) is defined as the expectation value of the projection operator... 

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