Strong exciton-phonon coupling incoherent excitons

4 Strong exciton phonon coupling incoherent excitons 

If we assume that the variation of the function P(m,f) over distances of the order of the lattice constant is small, then eqn (14.27) reduces to the diffusion equation. Indeed, assuming that the vector m varies continuously, we obtain 

If W(n, m) = W(m, n) (which holds if there is no static disorder in the crystal), substitution of the above expression into eqn (14.27) yields 

For crystals with several molecules (labeled by s) per unit cell, which have symmetry operations exchanging molecules with different s values, as holds for anthracene crystals, a similar procedure yields 

under the given conditions, the calculation of the diffusion tensor reduces to the determination of the hopping rates F(ns, ms ). 

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STRONG EXCITON-PHONON COUPLING INCOHERENT EXCITONS 419... 

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