Anisotropic Effects in Optical Excitation of Excitons in Molecular Crystals

Abstract

The expression for the energy of an exciton state contains certain dipole lattice sums. These sums are strongly dependent on the direction of the propagation vector of the exciton, even when the magnitude of this vector is very small. As a result, the energy of an optically-excited exciton may depend strongly on the direction of propagation of the absorbed radiation, if the excitation takes place coherently over a region whose linear dimensions are comparable with or much larger than a wavelength. The behavior of the lattice sums may also introduce a dependence of the exciton polarization and of the absorption coefficient on the direction of propagation. The magnitudes of these effects were calculated for the cases of the first and second transitions of naphthalene and of anthracene. Experimental observations of the extent to which these effects occur could provide information on the dimensions of the region of excitation of the exciton.