Optical spectra of molecular aggregates near the strong coupling limit

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Abstract
It is shown that the Green function operator for the electronic excited states of an arbitrary aggregate of identical monomers can be expanded in a series of terms in which the first represents the strong coupling (exciton) limit. Higher terms in this series successively improve the exciton limit by bringing in the effects of vibrational structure of the monomers. By evaluating the contribution to optical spectra of three terms in this series explicit expressions are found for the intensities, positions, and widths of individual exciton peaks near the strong coupling limit. It is shown for example that each exciton peak is narrowed with respect to the monomer band width at most by a factor of M−1/2 where M is the number of monomers in the aggregate. An approximate expression for optical absorption by molecular aggregates (DGS model) derived earlier is shown to satisfy the relations derived here.