Single-particle and collective excitations in ferromagnetic iron from electron-energy-loss spectroscopy

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Abstract
Electron-energy-loss spectra have been taken on clean and oxygen-covered polycrystalline iron samples. The results are discussed in terms of their dependence on the surface oxidation and on the primary electron energies. Bulk and surface collective excitations are identified. Single-particle interband transitions are found in excellent agreement with previous optical results. A new strong transition around 10 eV, not present in optical experiments, is identified as a dipole-forbidden but quadrupole-allowed process. Interband transitions are interpreted in terms of the band structure calculated by Callaway and Wang, which is thus checked up to much higher energies than previously possible. The agreement of the band structure with our experimental results can be improved at high energies by shifting the calculated conduction bands downward in energy by about 1 eV.