Band dispersion in the deep 1s core levels of graphene

It is generally assumed that electrons in deep atomic corestates are highly localized and do not participate in the bondingof molecules and solids. This implies well-defined core-levelbinding energies and the absence of any splitting and band-likedispersion, a fact that is exploited in several powerful experimentaltechniques, such as X-ray photoemission spectroscopy.Violations of this assumption have been found for only a fewsmall molecules in the gas phase such as C2H2 or N2 withmuch stronger bonding and shorter bonding distances thanpresent in solids. Herewe report the observation of a sizeableband-like dispersion of the C 1s core level in graphene, asingle-layer honeycomb net of carbon atoms that is attractingconsiderable attention at present in the scientific community.The dispersion is observed as an emission-angle-dependentbinding-energy modulation and it is shown that under appropriateconditions only the bonding or antibonding statescan be observed. A very similar dispersion is also found byab initio calculations.