Imaging molecules with the scanning tunneling microscope: A theoretical interpretation of benzene on platinum

The images of benzene on Pt(111), as obtained by the scanning tunneling microscope, have been calculated theoretically. The simulations show that the shape of the molecule pattern in the image is strongly dependent on the chemisorption site on the surface, in agreement with the experimental data. For a given site, the variations of the images with the geometric parameters of the adsorption are discussed. The internal structure of the pattern is enhanced when the molecule is more distorted or adsorbed closer to the surface. By a decomposition of the calculated image on different subsets of molecular orbitals, the shape of the hollow site image is analyzed in detail. It is shown that is is not possible to restrict the calculation to the pi orbitals of benzene and that the s orbitals built from the 2s atomic orbitals of carbon play an important role.