Symmetry dependence of vibration-assisted tunneling
Institute of Experimental and Applied Physics, University of Regensburg, 93053 Regensburg, Germany
The use of ultra-thin insulating films on metal substrates as a support for individual molecules allows for the almost unperturbed electronic properties of molecules to be studied by means of the scanning tunneling microscopy (STM) as it facilitates an electronic decoupling from the substrate. In this weak coupling regime, the current has to pass a double-barrier tunneling junction, and inelastic excitation of molecules manifests itself as vibronic satellites adjacent to electronic resonances in the differential conductance. Hence, this geometry enables spatially resolved vibronic spectroscopy . In usual STM-based inelastic electron tunneling spectroscopy , which is a regime quite different from the above mentioned, it has been realized that the symmetries of electronic wave functions play a crucial role .
Here we will discuss vibronic spectroscopy of individual pentacene molecules with sub-molecular resolution in such a double barrier tunneling junction geometry in detail. In this context, we show that the spatial position of the electron injection as well as the local electronic wave function symmetry dramatically affect the electron-vibron coupling. This is in contrast to the usual treatment of electron-vibron coupling in the Franck-Condon picture. We observe that the so-called reorganization energy spatially varies by more than a factor of two . Our finding can be rationalized in a simple model of vibration-assisted tunneling, which has analogies to optical excitations in indirect semiconductors and is not limited to STM experiments only. Finally, our recent effort to establish time-resolved inelastic excitation experiments by combining a pulsed THz laser source with low-temperature STM will briefly be discussed.
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