Character of Electronic States in the Transport Gap of Molecules on Surfaces

We report on scanningtunneling microscopy (STM) topographs ofindividual metal phthalocyanines (MPc) on a thin salt (NaCl) filmadsorbed on a gold substrate, at tunneling energies within the molecule'selectronic transport gap. Theoretical models of increasing complexityare discussed. The calculations for MPcs adsorbed on a thin NaCl layeron Au(111) demonstrate that the STM pattern rotates with the molecule'sorientations in excellent agreement with the experimental data.Thus, even the STM topography obtained for energies in the transportgap represent the structure of a one atom thick molecule. It is shownthat the electronic states inside the transport gap can be ratheraccurately approximated by linear combinations of bound molecularorbitals (MOs). The gap states include not only the frontier orbitalsbut also surprisingly large contributions from energetically muchlower MOs. These results will be essential for understanding processes,such as exciton creation, which can be induced by electrons tunnelingthrough the transport gap of a molecule.