Electron excitations in solid C60 : energy gap, band dispersions, and effects of orientational disorder

Electron excitation energies and photoemission spectra in undoped, solid C60 are calculated using a quasi-particle approach. The effects of orientational disorder and electron correlations are studied. We find a band gap of 2.15 eV, in good agreement with experiment, and about 1 eV widths for the highest occupied-molecular-orbital and lowest-unoccupied-molecular-orbital (LUMO) bands. Calculated angle-resolved, inverse photoemission spectra for the LUMO bands show very little angular dependence, explaining recent experimental work on epitaxial thin films. The present results suggest that undoped solid C60 is a standard band insulator. (Author (revised))