Ab Initio Theory of Photoemission from Graphene

Angle-resolved photoemission from monolayer and bilayer graphene is studied based on an ab initio one-step theory. The outgoing photoelectron is represented by the time-reversed low energy electron diffraction (LEED) state Phi(LEED)*, which is calculated using a scattering theory formulated in terms of augmented plane waves. A strong enhancement of the emission intensity is found to occur around the scattering resonances. The effect of the photoelectron scattering by the underlying substrate on the polarization dependence of the photocurrent is discussed. The constant initial state spectra I(k(parallel to), (h) over bar omega) are compared to electron transmission spectra T(E) of graphene, and the spatial structure of the outgoing waves is analyzed. It turns out that the emission intensity variations do not correlate with the structure of the T(E) spectra and are caused by rather subtle interference effects. Earlier experimental observations of the photon energy and polarization dependence of the emission intensity I(k(parallel to), (h) over bar omega) are well reproduced within the dipole approximation, and the Kohn-Sham eigenstates are found to provide a quite reasonable description of the photoemission final states.