Photoemission study of the electronic structure of C₆₀ and K_xC₆₀

High-resolution photoemission studies have been performed on undoped and K-doped C₆₀ films. In the undoped material, temperature-dependent measurements showed no significant change of the valence-band electronic structure on crossing the structural phase transitions at 90 K and 249 K. Studies of K_xC₆₀ conducted with a variety of photon energies and doping levels show that K₃C₆₀ is a metal, with a sharp Fermi edge situated at the maximum of the occupied part of the lowest unoccupied molecular orbital-or LUMO-derived bands. The density of states observed at the Fermi level shows a maximum at x=3, indicating that there is no pseudogap at the Fermi level of K₃C₆₀. The spectral weight of the occupied LUMO-derived states with respect to those of the highest occupied molecular orbital (HOMO) is found to be photon-energy dependent. A surface K content greater than x=6 can be achieved and is found to be correlated with the presence of a potassium oxide overlayer. The results are discussed with reference to the need for absolute, accurate determination of the surface K content of the samples studied by photoemission.