LUMO band of K-dopedC60single phases: A photoemission and yield-spectroscopy study

The LUMO-derived band of K-doped ${\mathrm{C}}_{60}$ was investigated by comparing the results of three measurements: yield spectroscopy and photoemission excited with hν=6.2 and 21.2 eV. Homogeneous single-phase near-surface layers were used in the investigation. The room-temperature line shape of the LUMO-derived band for the ${\mathrm{K}}_3$ ${\mathrm{C}}_{60}$ phase is established. The insulating character of the ${\mathrm{K}}_4$ ${\mathrm{C}}_{60}$ phase is confirmed. The presence of an appreciable spectral weight close to the Fermi level in the insulating phases is detected. Evidence against a simple one-electron description of the LUMO-derived band is discussed.