Unoccupied molecular orbitals of aromatic hydrocarbons adsorbed on Ag(111)

Abstract

The unoccupied π* bands of epitaxial overlayers of benzene, naphthalene, anthracene, tetracene, perylene, and coronene on a Ag(111) surface have been studied by angle resolved inverse photoemission spectroscopy. A comparison with HAM/3 MO calculations and electron transmission gas phase measurements enables the assignment of the π* affinity levels of these organic systems. In conjunction with previous angle resolved photoemission, optical absorption, and near edge x-ray absorption studies a complete picture of their valence band structure and morphology evolves. Optical and x-ray absorption excitation energies are discussed in the light of the experimentally determined one-electron correlation diagram. Gas phase–solid relaxation energies of the affinity levels are found to decrease in the series of benzene to tetracene (1.7→0.5 eV) attributed to the increasing size of the involved molecular orbitals. For benzene a low lying σ*-shape resonance has been identified in the isochromat inverse photoemission spectrum.