Identifying Molecular Orbital Energies by Distance-Dependent Transition Voltage Spectroscopy

Besides current
voltage spectroscopy, also transition voltage spectroscopy (TVS) becomes an interesting tool to investigate the energetic position of the molecular orbitals involved in the tunneling process. We used scanning tunneling spectroscopy to perform both spectroscopy techniques as a function of the tip
substrate distance. Employing our model system, benzoic acid on a Cu(110) surface, we could observe a step in the transition voltage using distance-dependent TVS. Combining the spectroscopic results with density functional theory based calculations, it was possible to identify the molecular orbitals responsible for charge transport. Moreover, it was found that different molecular orbitals are responsible for charge transport if varying STM tip
substrate distances are examined.