Experimental band gap and core-hole electron interaction in epitaxialC60films

This work employs a common experimental energy scale for photoemission, inverse photoemission, and x-ray absorption data, which is essential for a reliable experimental determination of energy separations (gaps) between occupied and empty states of solids. For epitaxial layers of ${\mathrm{C}}_{60}$ prepared on layered ${\mathrm{VSe}}_2$ single crystals we demonstrate a good agreement between experiment and previous quasiparticle calculations of the HOMO-LUMO distance and the band gap. Comparison of inverse photoemission and x-ray absorption data from the same sample on a common energy scale shows that conduction-band states cannot be derived from absorption data and enables us to determine the direct experimental value of the core-hole electron interaction U in solid ${\mathrm{C}}_{60}.$