ELECTRONIC-STRUCTURE OF MOLECULAR VAN-DER-WAALS COMPLEXES WITH BENZENE - IMPLICATIONS FOR THE CONTRAST IN SCANNING-TUNNELING-MICROSCOPY OF MOLECULAR ADSORBATES ON GRAPHITE

We investigate the electronic structure of molecular model systems in order to improve our understanding of the nature of the contrast, whic h is observed in the scanning tunneling microscopy (STM) imaging of or ganic adsorbates on graphite. The model systems consist of a benzene m olecule, representing the substrate surface, interacting with various molecules representing alkyl chains, oxygen- and sulfur-containing gro ups, fluorinated species, and aromatic rings. We perform quantum-chemi cal calculations to determine the geometric structure, stability, and electronic structure of these molecular complexes and analyze the theo retical results in relation with experimental STM data obtained on mon olayers physisorbed on graphite. It appears that the STM contrast can be correlated to the energy difference between the electronic levels o f the substrate and those of the adsorbate. Finally, we observe that t he introduction of a uniform electric field in the quantum-chemical mo deling can enhance the electronic interaction between the partners in the complex. (C) 1997 American Institute of Physics.