Small solutes, mode coupling theory

One should keep in mind that the rate constant increases monotonically with coupling only if the relaxation time of the fast mode is infinitely small. If it is finite, as it should be, the rate versus couphng dependence starts as the Golden Rule prescribes, then continues according to the Sumi-Marcus theory, but finally saturates at a certain value determined by the relaxation time of the fast mode. This effect is illustrated in Figure 9.18(h) for the case where both fast and slow modes relax exponentially. In this case, the problem is reduced to solution of a system of 2D diffusion equations along parabolic potential surfaces corresponding to two coordinates and... 

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