Inelastic processes versus diffraction effects: Polar-angle energy-loss spectra of the graphite K edge

We report the angular behavior of the reflection energy-loss features above the carbon K edge in highly oriented pyrolitic graphite. The intensity variations of the pi* and sigma* structures have been used to evaluate the fine details of the basic mechanism governing the interaction between primary electrons of low kinetic energy (similar to 500 eV) and a core electron transition. We have considered the most probable scattering events, namely: single inelastic scattering, energy loss preceded by diffraction, and diffraction preceded by energy loss. In our experimental condition we have evidenced that the main channel of interaction is essentially constituted by inelastic scattering followed by diffraction processes. Our experiment resolves the degeneracy proposed by theoretical calculations that assign an equal probability to the two transitions assisted by elastic scattering. The energy-loss process does occur at a very small scattering angle from the primary beam direction (parallel-q exchanged) and this explains the capability for low-energy electrons to investigate the orientational dependence of the density of states and the bond-length direction of an adsorbed surface atom as currently done in surface x-ray-absorption measurements.