Paradigm of Free Energy Surfaces

The CT/ET free energy surface is the central concept in the theory of CT/ ET reactions. The surface s main purpose is to reduce the many-body problem of a localized electron in a condensed-phase environment to a few collective reaction coordinates affecting the electronic energy levels. This idea is based on the Born-Oppenheimer (BO) separation of the electronic and nuclear time scales, which in turn makes the nuclear dynamics responsible for fluctuations of electronic energy levels (Eigure 1). The choice of a particular collective mode is dictated by the problem considered. One reaction coordinate stands out above all others, however, and is the energy gap between the two CT states as probed by optical spectroscopy (i.e., an experimental observable). 

Statistical average over the electronic degrees of freedom 

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The paradigm of free energy surfaces provides a very convenient and productive conceptual framework to analyze the thermodynamics and dynamics of electronic transitions in condensed phases. It, in fact, replaces the complex dynamics of a quantum subsystem interacting with a many-body... 

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