The theory of the dynamics of Frenkel excitons is discussed within the context of transient grating experiments. Our results are based on the stochastic model of Haken and Strobl. We show that the transient grating signal is very sensitive to the nature of the initial state prepared by the crossed excitation beams, especially when the exciton motion is ‘‘coherent’’ (when there is no exciton–phonon scattering). If the initial density matrix is assumed to be site diagonal, we find agreement with previous theoretical work of Wong and Kenkre. Our results are consistent with experiments performed by the Fayer group on anthracene, where it appears that the exciton motion is incoherent. However, based on their results and our theory, we suggest that it may be possible to observe partially coherent transport at T=2 K with a grating spacing of 2000 Å.

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