Theory of the interaction of light with large inhomogeneous molecular aggregates. I. Absorption

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Abstract
A general method to describe the spectroscopy of large, internally inhomogeneous particles is presented. The theory utilizes an approach similar to the one used by DeVoe in the treatment of the optical properties of polymers. The particle is divided into groups and the internal field is calculated by solving a self‐consistent set of linear equations in the field amplitude at each group in the particle. It is found that if the particle is dense the intermediate and radiation coupling mechanisms must be included in addition to the dipole–dipole coupling. Through these coupling mechanisms it is found that the excitation generated at each group in the chromophore can delocalize over regions comparable to the size of the wavelength of light. The response of the particle to the light can then be described in terms of ‘‘collective modes’’ of excitation of the entire particle, the amplitude of each mode being controlled by the geometrical relation between the groups and the efficiency in energy transfer between any two groups in the aggregate. The spatially averaged equations of the absorbance for a collection of large inhomogeneous arbitrarily shaped aggregates are derived.