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

Abstract

The energy dependence of the mean free path $\ensuremath{\lambda}(E)$ in graphite at low kinetic energies (below $\ensuremath{\sim}50\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$) 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 $\ensuremath{\lambda}(E)$, appearing as strong intensity minima in the Si $2p$ signal, is found to reflect band gaps in the unoccupied states of graphite. Furthermore, $\ensuremath{\lambda}(E)$ is derived based on analysis of very-low-energy electron diffraction data supported by calculations of the complex band structure of unoccupied states, where $\ensuremath{\lambda}(E)$ appears from the Bloch wave damping factor. Conceptually different, the two methods yield equivalent $\ensuremath{\lambda}(E)$. The strength of the diffraction structure in $\ensuremath{\lambda}(E)$ manifests a significant elastic contribution to electron scattering at low energies, sharply increasing in the band gaps of the unoccupied states.