Frenkel exciton model of electron energy loss spectroscopy in image-PTCDA

Recent experimental findings concerning electron energy loss spectroscopy in image-Perylene-tetracarboxylic-dianhydride are analysed in terms of a Frenkel exciton model. Taking into account the energy dispersion of excitations with finite momentum transfer, the image-dependence of the dielectric tensor and the corresponding electron energy loss functions can be calculated. The exciton dispersion with a minimum at image yields a red shift of the lineshape of loss functions at large k, as observed experimentally.