Resonance intensity of the n = 1 image potential state of graphene on SiC via two-photon photoemission

The interpretation of the n = 1 image potential state data on graphene on SiC is far from being clarified. In contrast with graphene grown on metallic substrates, graphene on SiC shows a very broad n=1 image state which is sometimes resolved in two different peaks. In literature the double feature has been ascribed to the presence of a second image state, due to the buffer layer when an incomplete graphene layer occurs, or, alternatively, has been identified as the double series of image states predicted for the free-standing graphene. Here, by varying the pump laser photon energy, in the non-linear photoemission experiment, we are able to reveal the presence of a resonance intensity of the image potential state that in principle could help to shed light on the origin of n=1 image state on graphene on SiC.