C 1s ionisation potential and energy referencing for solid C-60 films on metal surfaces

We present a measurement of the C Is ionisation potential (IP) for solid C-60, as well as a reliable determination of the highest occupied molecular orbital (HOMO) IP. The difference between the gas phase and solid state measurements for the HOMO agrees well with theory. We discuss these results with respect to the appropriate referencing of photoemission binding energies for monolayer, bilayer and multilayer films of C-60 on metal surfaces, Our measurements show that beyond the first chemisorbed monolayer, the binding energies of C-60 are most properly referenced to the vacuum level in a manner reminiscent of noble gases on metal substrates, and consistent with the identification of C-60 as a molecular solid. For thicker films we find additional energy shifts, which we attribute to steady-state charging. We use our results to correct a previous determination of the intermolecular electron interaction in solid C-60.