Scattering angle dependence of the surface excitation probability in reflection electron energy loss spectra

Reflection electron energy loss spectra (REELS) of medium energy electrons backreflected from a tungsten surface were measured. The incident and exit angles were varied over a large angular range keeping the scattering angle fixed. Two scattering geometries were studied corresponding to situations where the distribution of pathlengths of the electrons near the surface are significantly different. The spectra were decomposed into contributions of surface and volume excitations. It was found that the surface excitation probability depends not only on the incident and emission angle, but also on the scattering angle. This implies that elastic deflections in the surface scattering zone play a significant role in an energy loss experiment. Thus, the path of the reflected electrons crossing the surface scattering zone is not generally rectilinear. In consequence, plural surface scattering is not governed by the Poisson stochastic process.