Urbach rule in the optical spectra of crystalline and amorphous organic-solids

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In this paper we explore the effects of microscopic structural disorder on the low energy optical absorption edges in some crystalline and amorphous organic solids. We advance a theoretical model which incorporates the simultaneous effects of static disorder and of exciton-phonon coupling on Frenkel exciton states. Explicit theoretical expressions were derived for the self energy and for the absorption lineshapes by the introduction of a two-step effective-field approximation, defining an effective exciton-phonon hamiltonian which was subsequently utilized within the framework of a single site average t-matrix approximation for disorder scattering. Numerical results for the low energy lineshapes over a broad temperature range demonstrate that exciton scattering by a static disorder field results in Urbach type low energy absorption tails, where the Urbach slope is temperature independent.

Original languageEnglish
Pages (from-to)421-430
Number of pages10
JournalChemical Physics
Issue number3
StatePublished - 15 Dec 1977


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