Photoemission study of the electronic structure of ${\mathrm{C}}_{60}$ and ${\mathrm{K}}_{\mathit{x}}$${\mathrm{C}}_{60}$

High-resolution photoemission studies have been performed on undoped and K-doped ${\mathrm{C}}_{60}$ 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 ${\mathrm{K}}_{\mathit{x}}$ ${\mathrm{C}}_{60}$ conducted with a variety of photon energies and doping levels show that ${\mathrm{K}}_3$ ${\mathrm{C}}_{60}$ 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 ${\mathrm{K}}_3$ ${\mathrm{C}}_{60}$. 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.