Abstract
The primary electronic spin relaxation mechanism for triplet excitons in many molecular crystals arises from hopping transport between two orientationally inequivalent sites and is the source of both EPR linewidth and level equilibration. A generalized stochastic theory of resonance linewidth due to Blume which was previously applied to this mechanism and shown in its random-phase approximation (RPA) to yield the exciton EPR linewidth spectrum is here shown to give a complete formulation of both linewidth and equilibration rate. The method employs the averaged time-development superoperator of Blume's theory in the RPA in order to construct an equation of motion for the appropriately averaged spin-density matrix from which these two can be extracted. We show associations between this work and related studies by Kubo and Suna. Comparison is made between rates calculated for anthracene and values of ${T}_{1}$ deduced by Haarer and Wolf from a Bloch analysis of their EPR saturation measurements.
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