Correlation functions nonexponential behavior

There should exist a correlation between the two time-resolved functions the decay of the fluorescence intensity and the decay of the emission anisotropy. If the fluorophore undergoes intramolecular rotation with some potential energy and the quenching of its emission has an angular dependence, then the intensity decay function is predicted to be strongly dependent on the rotational diffusion coefficient of the fluorophore.(112) It is expected to be single-exponential only in the case when the internal rotation is fast as compared with an averaged decay rate. As the internal rotation becomes slower, the intensity decay function should exhibit nonexponential behavior. 

Figure 3. Dipolar J, for a CH carbon as a function of an effective correlation time for molecular tumbling, t. Key A, isotropic or pseudoisotropic tumbling characterized by an exponential autocorrelation function G(t) and B, complex behavior resulting from a nonexponential decay of G(t). This requires that motion be represented by a set of t s, or a distribution around some mean. (Reproduced, with permission, from Ref. 20. Copyright 1981, Adenine Press.)...

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