Lateral confinement of image electron wave function by an interfacial dipole lattice

Image-potential states on Cu(111) surfaces covered by thin films of C60 fullerene have been characterized by angle-resolved two-photon photoemission spectroscopy. Metal-to-molecule electron transfer within the first layer creates a 4×4 superlattice of surface dipoles. We show that such a surface dipole lattice provides lateral confinement of image-electron wave functions. Measurements of parallel dispersion indicate that the n=1 image state is localized in the presence of one monolayer of C60 but becomes delocalized by the addition of a second layer. Quantum mechanical calculations explain this in terms of the screening of the dipole potential, thus, restoring the free-electron behavior parallel to the surface. These results show that a surface dipole lattice can effectively control the interfacial electronic structure.