Reversible Phase Transformation and Doubly Charged Anions at the Surface of Simple Cubic RbC(60)

The simple cubic phase of a RbC(60) 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 RbC(60). A molecular charge state is identified in the valence band and core-level photoemission spectra which arises from C(60)(2-) anions and contributes to the spectral intensity at the Fermi level.