  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.
