Effective energy loss functions of Mo and Ta derived from reflection electron energy loss spectra

Effective energy loss functions (EELF) of Mo and Ta were derived from reflection electron energy loss spectroscopy (REELS) spectra measured at different primary energies using extended Landau approach. The plasmon behaviors and fine structures of energy losses presented in the energy loss function are analyzed. For EELF of Mo, the peak at 9.9 eV is a composition of bulk plasmon at 10.4 eV and surface plasmon at 9.5 eV, and the other peak at 22.4 eV is made of bulk plasmon at 24.4 eV and surface plasmon at 19.8 eV. As for Ta, the peak at 19.7 eV is contributed by bulk plasmon at 20.8 eV and surface plasmon at 8.4 eV. The EELF thus includes both the surface effects and bulk excitation, where surface excitation is important mainly at low energy losses; the competition between surface and bulk excitation modes is analyzed for varied primary energy. The Monte Carlo simulated REELS spectra with the derived EELF agree well with the experimental results.