Photon energy dependence of angle-resolved photoemission spectroscopy in graphene

The photon energy dependence of angle-resolved photoemission spectroscopy (ARPES) in graphene is investigated experimentally and theoretically. By applying light with energy of around 46 eV, we found an unexpected increase in the ARPES relative intensity of graphene for the p branch (ARPES spectra brightened by the p-polarized light) with respect to the s branch (those brightened by the s-polarized light). The origin of the enhanced p-branch intensity is explained by first-principles calculations, in which we show (1) the optical dipole vector as a function of final-state energies of the excited electron, (2) the absorption intensity as a function of the incident light angle, and (3) the symmetry of the initial and the final states. The calculated results imply that the dipole vector of the excited electron near 46 eV has an exceptionally large component in the normal direction of the graphene surface compared to that within the graphene plane, which could be the main reason for the enhancement of the p-branch intensity.