Elastic scattering effects in the electron mean free path in a graphite overlayer studied by photoelectron spectroscopy and LEED

The energy dependence of the mean free path lambda(E) in graphite at low kinetic energies (below similar to50 eV) is studied using the synchrotron radiation excited Si 2p core level photoemission signal from a SiC substrate attenuated by an epitaxial graphite overlayer. Diffraction structure in lambda(E), appearing as strong intensity minima in the Si 2p signal, is found to reflect band gaps in the unoccupied states of graphite. Furthermore, lambda(E) is derived based on analysis of very-low-energy electron diffraction data supported by calculations of the complex band structure of unoccupied states, where lambda(E) appears from the Bloch wave damping factor. Conceptually different, the two methods yield equivalent lambda(E). The strength of the diffraction structure in lambda(E) manifests a significant elastic contribution to electron scattering at low energies, sharply increasing in the band gaps of the unoccupied states.