REEL spectra from aluminium: experiment and Monte Carlo simulation using two different dielectric functions

Within the framework of the dielectric theory, the spectral response of a solid irradiated by an electron beam is described by the energy loss function. In the limit of zero momentum transfer, the dielectric function is generally known from optical data for manymaterials. The main problem, when electron beams are involved, concerns the extension of the dielectric function from the optical limit to non-zero momentum transfer. This extension requires introducing a dielectric function, which depends on both energy loss andmomentum transfer. Using Al–representative of a dilute gas of free electrons–as a case study, the present work compares two Monte Carlo (MC) simulations with experimental reflection electron energy loss (REEL) spectra excited by 2 keV electrons. To obtain the two MC simulated spectra, the dielectric function was modelled, respectively, according to Ashley (1988) and according to an early suggestion by Ritchie and Howie (1977), later reconsidered by Yubero and Tougaard (1992). MC simulated REEL spectra obtained according to the latter model reproduce the experimental REEL spectra better than those obtained using the Ashley dielectric function. The comparison involves not only the single scattering plasmon peak (bulk and surface) but also multiple scattering peaks.