  The simple cubic phase of a RbC60 thin film has been studied using
photoelectron spectroscopy. The simple cubic-to-dimer transition is found to be
reversible at the film surface. A sharp Fermi edge is observed and a lower
limit of 0.5 eV is found for the surface Hubbard U, pointing to a
strongly-correlated metallic character of thin-film simple cubic RbC60. A
molecular charge state is identified in the valence band and core level
photoemission spectra which arises from C602- anions and contributes to the
spectral intensity at the Fermi level.
