Probing the non-analytic properties of exciton band structures by reflection spectroscopy

A method of probing the exciton band structure about the centre of the Brillouin zone is developed. Oblique angle reflectance spectra at angles of incidence up to 80 degrees for a (001) face of an anthracene crystal have been obtained for both an air-crystal and a silica-crystal interface. A method for transforming the reflectance dam to optical constants is developed and used. From this method the direction of propagation of radiation (k) over cap at any frequency inside the crystal for any incident angle can be determined. For a model of an isotropic crystal consisting of anisotropic molecules, dipole-dipole theory for an arbitrary (k) over cap is developed, exciton band structures from this theory are calculated and theoretical spectra based on these calculated band structures arc presented. The theoretical spectra are in very good agreement with the experimental data. Information about the exciton density of states is deduced from the results.