Diffuse Unoccupied Molecular Orbital of Rubrene Causing Image-Potential State Mediated Excitation

Abstract

We show the significant role of the Rydberg-like unoccupied molecular orbital in the photoexcitation process at the rubrene/graphite interface. Two-photon photoemission (2PPE) spectroscopy revealed a prominently strong resonant excitation of an unoccupied molecular orbital (MO) at 3.26 eV above the Fermi level (J. Phys. Chem. C 2012, 116, 5821). The excitation was attributed to be mediated by the image potential state (IPS) on graphite. We investigate in this paper the nature of the MO by 2PPE spectroscopy and density functional theory (DFT) calculations. The DFT calculation involving diffuse atomic orbitals predicts a nearly cylindrical unoccupied MO extending over four phenyl rings at the energy of the interest. Polarization dependence of 2PPE spectroscopy shows that the unoccupied MO is nearly symmetric to the plane of the light incidence in accordance with the calculated MO. The two-dimensional free electron wave function of the IPS can interact with the MO that has no node within the molecular framew...