Surface Excitons in Crystal Anthracene

Normal incidence reflection spectra from artifically surface roughened an thracene crystals probed on the (ab) face provide evidence for a roughness induced surface molecular exciton. The frequency at which the resonance associated with that surface exciton appears is predicted by a theory valid in the macroscopic dielectric formalism of the effects of surface roughness on the bulk modes. It is suggested that surface roughness introduces new boundary conditions for the Maxwell equations so that the surface modes have some radiative character. For an unroughened crystal surface, a frustrated total reflectance experiment could detect surface excitons and an attempt to do this is described. These experiments lead to a clearer understanding of the role of the surface in determining the optical and other properties of crystal anthracene.