Analysis of the anisotropy of excitons in pentacene single crystals using reflectivity measurements and electron energy-loss spectroscopy

The dielectric properties of pentacene single crystals have been studied using a combination of reflectivity measurements and electron energy-loss spectroscopy. The low lying electronic singlet excitations are significantly anisotropic within the reciprocal a*, b* plane. Our results allow the quantitative analysis of the dielectric function in dependence of the light polarization. We discuss the experimental results with respect to the microscopic understanding of the electronic excitations. In particular, our results indicate that a description of the energetically lowest electronic excitations of pentacene based on molecular Frenkel excitons is incomplete.