Exchange-excited f-f transitions in the electron-energy-loss spectra of rare-earth metals

Dipole-forbidden f-f multiplet excitations are found to characterize the electron-energy-loss spectra of Pr, Nd, Sm, Gd, Tb, Dy, Ho, and Er when the primary-electron energy is close to the binding energy of the 4d electrons. In Sm and Gd the f-f transitions are also detected at lower primary energies. The identification of the sharp peaks observed in the spectra as f-f excitations comes from straightforward comparison with the absorption spectra of the trivalent ions. The excitation energies of the f-f transitions are found to depend very weakly on the chemical environment. The exchange nature of the f-f excitation process explains the strong decrease of the intensities of the f-f peaks when the primary energy is raised, and is consistent with the resonant enhancement observed for primary energies near the 4d-4f threshold. A similar resonance is also observed in the primary-energy dependence of the intensity of the plasmon peak.