Band dispersion in the deep 1s core level of graphene

Chemical bonding in molecules and solids arises from the overlap of valence electron wave functions, forming extended molecular orbitals and dispersing Bloch states, respectively. Core electrons with high binding energies, on the other hand, are localized to their respective atoms and their wave functions do not overlap significantly. Here we report the observation of band formation and considerable dispersion (up to 60 meV) in the $1s$ core level of the carbon atoms forming graphene, despite the high C $1s$ binding energy of $\approx$ 284 eV. Due to a Young's double slit-like interference effect, a situation arises in which only the bonding or only the anti-bonding states is observed for a given photoemission geometry.