Polarization of C₆₀ by the surface electric field of GeS(001)

The polarisation of C₆₀ molecules on GeS(001) by the static electric field generated by the surface ions has been calculated using a simple model for molecular polarization with a local anisotropic polarizability for each carbon atom. The resulting molecular dipole is of the appropriate magnitude to explain the difference between the experimental surface dipole, and the calculated surface induced dipole upon adsorption of the C₆₀ molecule. However the results imply that the C₆₀ molecules sit above the close-packed rows of Ge ions in the surface, not in the shallow troughs of the surface corrugation. This can be explained in terms of the observed delocalisation of the C₆₀ electrons towards the surface resulting in an additional attraction (repulsion) of the molecule to the Ge cations (S anions) beyond the purely van der Waals interaction, which also accounts for the observed increased molecule-substrate bonding relative to the intermolecular bonding.