Reversible phase transformation and doubly-charged anions at the surface   of simple cubic RbC60
  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. 
Roberto Macovez, Andrea Goldoni, Luca Petaccia, Ingrid Marenne, Paul   A. Bruehwiler and Petra Rudolf