Momentum-resolved photoelectron absorption in surface barrier scattering on Ir(111) and graphene/Ir(111)

Time-of-flight momentum microscopy reveals sixfold symmetric sharp features of decreased intensity (dark lines) in constant-energy maps for clean Ir(111) and graphene/Ir(111). The dark lines have been observed for p- and s-polarized light in the photon-energy range of 20-27 eV and result from scattering of photoelectrons at the surface potential barrier. The phenomenon is strongly related to threshold effects in low-energy electron diffraction. A quantitative analysis of the dark lines' positions shows that the relevant reciprocal-lattice vector corresponds to the lattice of the topmost layer (in our case graphene and Ir, respectively). The dark lines appear in the momentum patterns only in a certain photon-energy range satisfying the additional condition that the electron wavelength matches the lattice periodicity.