Band dispersion in the deep 1s core level of graphene

It is generally assumed that electrons in deep atomic core states are highly localized and do not participate in the bonding of molecules and solids. This implies well-defined core-level binding energies and the absence of any splitting and band-like dispersion, a fact that is exploited in several powerful experimental techniques, such as X-ray photoemission spectroscopy. Violations of this assumption have been found for only a few small molecules in the gas phase such as C2H2 or N2 with much stronger bonding and shorter bonding distances than present in solids1, 2. Here we report the observation of a sizeable band-like dispersion of the C 1s core level in graphene, a single-layer honeycomb net of carbon atoms that is attracting considerable attention at present in the scientific community3. The dispersion is observed as an emission-angle-dependent binding-energy modulation and it is shown that under appropriate conditions only the bonding or antibonding states can be observed. A very similar dispersion is also found by ab initio calculations.