Calculation of the Refractive Index of Crystalline Anthracene in the Dipole Approximation

Abstract

The refractive indicies of crystalline anthracene are calculated near the long-wavelength edge of the first singlet transition using a classical theory of molecular excitons and the dipole approximation. Dipole sums and isolated molecule data from previous Davydov splitting calculations are used to test whether theories which predict numbers close to the experimental splitting also yield reasonable values of the refractive index. Agreement between the calculated and experimental indices is poor. This implies the existence of high-energy transitions of considerable total intensity above 6 eV which are the source of a considerable background polarization in the crystal. Intense transitions at high energy occur in many hydrocarbons and, by means of a collective (cooperative) interaction, could be responsible for a major part of the Davydov splitting of the lowest singlet transitions of moderate intensity (f≃0.1). This point is discussed in detail together with other factors influencing the size of the Davydov splitting in molecular crystals.