Shape of molecular adsorbates in STM images: A theoretical study of benzene on Pt(111)

The scanning tunneling microscope (STM) images of benzene on Pt(lll) have been calculated with different adsorption sites (hollow top, and bridge sites). Our aim was to get a qualitative understanding with a molecular-orbital (MO) approach of the factors that govern the STM image pattern and shape in the case of a molecular adsorbate. The calculated images strongly depend on the chemisorption site and they allow the assignment of each experimental image of benzene to a given site and orientation of the molecule. The contributions to the tunnel current of each molecular orbital were calculated and analyzed with the help of a simple analytic model of tunneling through a molecule. It is not only the orbitals close to the Fermi level that have a significant contribution to the current. Indeed, the shape of the orbital (especially the number of nodal planes perpendicular to the surface) also has a great importance. The final image results from the electronic interferences between these individual MO contributions and with the direct tip surface electronic current, as explained by the model. The main interference effect is between the sigma and the pi orbitals of benzene with a given symmetry, since these orbitals have a different phase behavior across the tunnel junction (respectively, symmetric and antisymmetric). The site differentiation in the STM pattern results from the effective symmetry of the adsorption site and, for the hollow case, only appears after interference of the MO contributions.