Reflection Electron Energy Loss Spectra beyond the optical limit

A position-dependent Inverse Inelastic Mean Free Path (IIMFP), calculated according to the Chen-Kwei theory of inelastic scattering, is used to obtain Reflection Electron Energy Loss Spectra (REELS) by a Monte Carlo approach. The basic ingredient of the theory is the energy-loss and momentum-transfer dependent dielectric function, for which we use a plasmon-pole approximation to the Lindhard dielectric function. The dependence on the momentum transfer enters by assuming a proper dispersion relation for the energy loss. Experiments reveal however that, beside energy loss, also plasmon damping disperses with momentum transfer. By comparing measured and calculated REEL spectra, we explore the role of damping dispersion for a quasi-free-electron material like silicon.