Effects of the internal field in the spectra of molecular crystals and the theory of excitons

It is shown that a very large set of optical properties of molecular crystals (pure crystals, crystals containing impurities, and crystalline solutions) in the region of the lowest singlet electronic (or vibrational) excited states, where the mutual interaction of the molecules does not violate their neutrality, can be treated without bringing in exciton concepts, but by generalizing to anisotropic media the methods of molecular optics that had been developed before Frenkel's ideas on the exciton had appeared. This treatment is based on using the method of the acting field, which goes back to Lorentz, and also on the results of Born and Ewald et al., which permit one to calculate the electric field acting on some particular molecule in the crystal, and in particular, with account taken of the higher multipoles of the polarizability of the molecules. It is shown that this approach permits one to calculate the dielectric constant of the crystal with account taken of spatial dispersion, to treat the polarization and splitting of light-absorption bands in crystals, and polarization of impurity absorption bands, as well as an entire series of other problems that were previously studied in less general form only within the framework of the Frenkel exciton theory. The limits of applicability of the presented approach are also discussed.