  We report on scanning tunneling microscopy (STM) topographs of individual
metal phthalocyanines (MPc) on a thin salt (NaCl) film on a gold substrate, at
tunneling energies within the molecule's electronic transport gap. Theoretical
models of increasing complexity are discussed. The calculations for MPcs
adsorbed on a thin NaCl layer on Au(111) demonstrate that the STM pattern
rotates with the molecule's orientations - in excellent agreement with the
experimental data. Thus, even the STM topography obtained for energies in the
transport gap represent the structure of a one atom thick molecule. It is shown
that the electronic states inside the transport gap can be rather accurately
approximated by linear combinations of bound molecular orbitals (MOs). The gap
states include not only the frontier orbitals but also surprisingly large
contributions from energetically much lower MOs. These results will be
essential for understanding processes, such as exciton creation, which can be
induced by electrons tunneling through the transport gap of a molecule.
