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Klaus Schulten. Ensemble averaged spin pair dynamics of doublet and triplet molecules. Journal of Chemical Physics, 80:3668-3679, 1984. Analytical expressions are presented which describe the spin dynamics of random ensembles of pairs of molecules, the partners being either of doublet ($^{2}D$) or of triplet ($^{3}T$) type. The spin dynamics are induced either by the hyperfine coupling ($^{2}D$) or by the zero-field splitting ($^{3}T$). The three spin transitions $^{1}$($^{2}D_{1}$ + $^{2}D_{2}$) $\rightarrow$ $^{3}(^{2}D_{1}$ + $^{2}D_{2}$), $^{4}(^{3}T_{1}$ + $^{2}D_{2})$, and $^{3,5}$($^{3}T_{1}$ + $^{3}T_{2}$) are considered at low and high magnetic fields. The ensemble average is taken over all nuclear spin configurations ($^{2}D$) or molecular reorientations by rotational diffusion or exciton migration in random matrices ($^{3}T$). The results obtained demonstrate that magnetic fields influence the processes of triplet quenching by radicals and triplet-triplet annihilation and thereby can provide information on the rotational diffusion of triplet molecules, exciton migration, and paramagnetic-diamagnetic exchange. Such effects predicted earlier by us for processes involving doublets [J. Chem. Phys. 71, 1878 (1979)] and discussed here, were recently demonstrated experimentally [F. Nolting, H. Staerk, and A. Weller, Chem. Phys. Lett. 88, 523 (1983)]. The results presented are also of general interest as the spin systems considered provide a solvable model to describe the degree of randomization in ensembles of quantum systems. Full Text

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