Direct visualization of diffuse unoccupied molecular orbitals at a rubrene/graphite interface.

Spectroscopic and nanoscale imaging investigations concerning the spatial extent of molecular orbitals at organic/substrate interfaces have been of intense interest to understand charge dynamics. Here, the spatial extent of unoccupied molecular orbitals of ultrathin rubrene [5,6,11,12-tetraphenyltetracene] films has been investigated with scanning tunneling microscopy and spectroscopy. Based on constant-current distance (z)-voltage (V) measurements, the unoccupied energy levels are elucidated and found to be consistent with previously reported macroscopic two-photon photoemission (2PPE) spectroscopy. In the diffuse unoccupied molecular orbitals reported with 2PPE (J. Phys. Chem. C, 2013, 117, 20098), nanoscale dz/dV spatial maps reveal that the local density of states of the orbitals extends over the rubrene molecules. Delocalization is also observed for the image potential states, which are inherently free-electron-like. This is in contrast to the localized nature of other unoccupied molecular orbitals. A nanoscale understanding of diffuse and delocalized molecular orbitals provides a fundamental insight into low-lying Rydberg states in polycyclic aromatic hydrocarbons.