Electronic surface reconstruction and correlation in the fcc and dimer phases of RbC60

A combined photoemission and x-ray absorption study of a RbC60 crystalline film is presented. We find evidence for an electronic charge reconstruction of the film surface in both the fcc and the dimer phases of RbC60. We confirm the previous conclusion on less crystalline films that the dimer phase is insulating. Several observations, such as the presence of molecular features in the photoemission spectra, indicate that at least partial electron localization occurs in the high-temperature fcc phase. In the fcc phase, the surface consists in a half-charge C-60 (111) plane and appears weakly metallic, as found for the bulk. In the dimer phase, the charge reconstruction simply implies the presence of neutral C-60 in the surface layer. The identification of neutral molecules in the surface layer drastically improves the agreement between calculations of the electronic density of states and photoelectron spectra in both phases.