Pentacene films grown on a graphite surface has been studied by using High-Resolution Electron-Energy-Loss Spectroscopy (HREELS) as a function of film thickness and of sample temperature. The HREELS spectra show that the adsorbed pentacene molecules are in a flat-lying geometry with the molecular plane parallel to the substrate. Further, we discuss the thickness and temperature dependences of the vibrational energies in terms of molecule-substrate and intermolecular interactions.
