STM-based inelastic electron tunnelling spectroscopy (IETS) is a powerful technique for the analysis of adsorption on a single molecular level. However, for complex molecules, the interpretation of IETS data is far from straightforward, because often only a small fraction of all vibrational modes is observed in the inelastic spectra, and it is difficult to predict a priori which of the modes should have a strong inelastic tunnelling cross section. On the other hand, inelastic scattering is an important issue in transport through single molecules.
In this contribution we use the commensurate PTCDA/Ag(111) interface to study the inelastic tunnelling process in the case of a complex molecule. The results of IETS are compared with HREELS data and theoretically calculated spectra.
Because the molecule adsorbs flat on the surface, we can study in detail how the inelastic cross section of some of the modes which we observe varies across the molecule. On the basis of this information and a simple symmetry analysis, a few vibrational modes visible in the IETS spectra can be identified. Moreover, we study the influence of the molecule-substrate interaction on the inelastic cross sections. Remarkably, slight variations of the adsorption conditions are found to enhance IETS signal by an order of magnitude. The origin of this enhancement is discussed.
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