In this paper we explore the effects of offhyphen;diagonal disorder on electronic energy transfer (EET) in an impurity band of an isotopicallyhyphen;mixed, organic solid at low temperatures. We have considered the localization of an elementary excitation in a system characterized by both diagonal disorder, originating from inhomogeneous broadeningWof the sitehyphen;excitation energies, and of offhyphen;diagonal disorder arising from the energetic spread sgr; of the transfer integrals. We have utilized an exact expression for the selfhyphen;energy of a disordered system where both the sitehyphen;excitation energies and the transfer integrals are characterized by a Lorentzian distribution, together with the localization function method of Liciardello and Economou to establish the localization condition in the center of the impurity band. Model calculations were performed for a Bethe lattice and for the Hubbard density of states, demonstrating the enhancement of delocalization due to offhyphen;diagonal disorder, whereupon the Anderson transition (AT) will be exhibited at higher values ofWthan in the original Anderson model (OAM), when sgr;=0. For large values of the ratioW/sgr;gsim;12 the effects of offhyphen;diagonal disorder are negligible. Numerical calculations of sgr; were performed for a random distribution of impurities, whileWwas roughly estimated for recent spectroscopic measurements. These data, together with the results of the model calculations for a Bethe lattice, established the existence of the critical impurity concentration quest; for EET in the impurity band. Offhyphen;diagonal disorder results in the lowering of quest; relative to the OAM; however, the effect of diagonal disorder is dominant in determining the termination of EET in the impurity band.
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