Suppression and enhancement of the exciton-phonon inter-action in optical absorption spectra of Frenkel exciton microcrystallites

A theory is presented of suppression and enhancement of the exciton-phonon interaction, which strongly affects optical absorption spectra in microcrys tallites. The motivation of the study comes from an experimental observatio n that the absorption bandwidth of aromatic microcrystallites increases wit h the crystallite size, but it shows a sudden fall to zero at a certain siz e. The present theory shows that the fall comes from the broadening of the exciton states due to phonon scattering, which is efficient only in crystal lites larger than a critical size. The broadening results in a remarkable r eduction of the absorption spectral peak height except in the lowest energy one. For the calculation of this problem the Fermi golden rule was not use d because the density of states cannot be defined, so direct one-by-one sum mation was carried out over terms which depend on discrete energy levels. ( C) 1999 Elsevier Science B.V. All rights reserved.