Dresden 2020 – scientific programme

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O: Fachverband Oberflächenphysik

O 15: Organic Molecules on Inorganic Substrates II: Mainly Porphyrins

O 15.10: Talk

Monday, March 16, 2020, 17:30–17:45, REC C 213

Vibrational excitation mechanisms in tunneling spectroscopy beyond the Franck-Condon model. — •Gaël Reecht, Nils Krane, Christian Lotze, and Katharina J. Franke — Freie Universität, Berlin, Germany

Since two decades, scanning tunneling spectroscopy (STS) is used to study vibrations of individual molecules on surface, either via inelastic tunneling spectroscopy (IETS) or vibronic spectroscopy. In the latter cases, experimental results were generally explained within the Franck-Condon picture, which treats vibronic excitations within the Born-Oppenheimer approximation and derives the excitation probabilities from an overlap integral of initial and final state. However this model cannot justify any spatial dependence of vibronic excitations. Here we study with low temperature STM, large organic molecules (thiophene derivative and phthalocyanine) on single layer MoS₂ on Au(111). The van der Waals layer acts as an efficient decoupling layer from the metal substrate and provides exceptional energy resolution of a few meV, which gives access to the rich vibronic signature of the investigated molecules. To explain the complete excitation model, spatially and energetically, we show that the Franck-Condon picture has to be combined with a vibration-assisted process, which perturbs the molecular wave functions, and therefore its overlap with the tip wave function. Using DFT calculations of the molecule in gas phase, our simple model reproduces the experimental spectra at arbitrary position of the STM tip over the molecule in great detail.