atom. The effect can be of considerable size, several hundred meV for photon energies of several keV, but its magnitude should be insignificant for the low energies used here. One would not expect it to lead to any periodic modulation either. 

This leaves a third possibility which is an initial state effect, i. e. a band-like dispersion of the initial state. The simplest conceivable picture for this is the formation of a σ -type band between the 1 s states of the two atoms in the unit cell of graphene, highlighted in orange in the sketch of Fig. 1. A tight-binding calculation of such a band is shown in Fig. 3(a). The absolute binding energy and the band width are arbitrarily chosen to mimic those observed here. The dispersion shows two bands with the highest energy separation at ¯ Γ and degeneracy at the ¯ K point of the two-dimensional Brillouin zone. In the following we refer to these bands, somewhat loosely, as the bonding and the anti-bonding band. 

On the face of it, it appears hard to reconcile such a dispersion with the experimental observation, as one would expect to observe a single, narrow C 1 s peak at ¯ K and a broad or even split peak at ¯ Γ. This is clearly not supported by the data. Also, the σ -band should be periodic in reciprocal space, e. g. the peak position and width should be the same at all ¯ Γ points. According to the right panel of Fig. 2, this is not the case either. Most of the ¯ Γ points marked in the figure appear close to either a maximum or a minimum in the binding energy but clearly the observed periodicity is not the same as that of the reciprocal lattice. 

The hypothesis of band dispersion and the experimental data can, however, be reconciled when taking into account a curious interference effect which is caused by the presence of two atoms in the unit cell of graphene. In the most simple picture, this interference can be interpreted as a type of Young’s double slit phenomenon in which the two atoms in the unit cell act as electron sources. The effect has been studied in detail for the valence band of graphite and graphene [4, 5]. 

As an illustration of the interference effect, we have calculated the expected photoemission intensities for all the anti-bonding and all the bonding states. The results are given in Fig. 3(b) and (c), respectively. The strength of the interference effect is evident: in the first Brillouin zone, for instance, emission from the anti-bonding states is entirely suppressed while it is intense from the bonding states. In the neighboring zones it is the other way round. Figs. 3 (d)-(f) show the emission intensity for smaller energy windows at the top of the anti-bonding band, at the bottom of the bonding band, and just below the Dirac point at ¯ K . In the last case, a constant energy contour shows a triangular shape, as expected for 

