Strong electron-phonon coupling at a metal/organic interface: PTCDA/Ag(111)

We analyze the electronic interaction of PTCDA, a ribbon-shaped functionalized polycyclic molecule commonly used as model system, with various silver single crystal surfaces, using high resolution electron energy loss spectroscopy as a probe. Special emphasis is placed on the coupling of hybrid electronic states to intramolecular vibrations. Under certain circumstances, this coupling leads to an extremely strong activation of otherwise forbidden vibrational modes. Apart from a strong intramolecular electron-phonon interaction, the formation of a metallic (i.e., partially filled) molecular band is a prerequisite for this effect, which is reported here both for the PTCDA/Ag(111) interface and for ultrathin potassium-doped PTCDA films on Ag(110) while undoped PTCDA/Ag(110) does not show this behavior. In the second part of the paper, we formulate and apply a theoretical model for the interfacial electron-phonon coupling, with the goal to understand the observed Fano line shapes in the case of PTCDA/Ag(111). This model allows us to accurately describe the observed line shape and trace it back to a nonadiabatic interaction between electronic states and molecular vibrations, secondly to rationalize why the K/PTCDA/Ag(110) system exhibits adiabatic behavior, and thirdly to extract values for the intramolecular electron-phonon coupling in PTCDA which are in good agreement with other data. The implications of our experimental observations and the theoretical analysis are discussed.