Abstract
Electron energy-loss spectra (at 75keV) of single crystal aluminum and silicon foils of [001] orientation were obtained by using an electron microscope (Hitachi HU-11A)-electron spectrometer (Wien filter) system. The operation of this apparatus has been described previously. In the present experiment, the microscope was operated in the selected-areadiffraction mode and the energy-loss spectra were recorded on photographic plates and subsequently obtained data by microdensitometry.Figure 1 shows the loss spectrum in aluminum with momentum transfer q in the <100> direction. In addition to the usual bulk plasmons at 15eV and surface plasmons at 6.7eV, two dispersive peaks below the bulk plasmon peak are also observed. The first peak which is relatively sharp starts linearly from 1.5eV at q∼0 and levels off around 9eV at the zone boundary (q = 1. 55 Å-1) . This peak is due to the excitation of direct non-vertical interband transitions along the X-W-X direction of energy bands in Al. The second peak, broad and diffuse with energy higher than the first one, has approximately linear dispersion which extends beyond 12eV and shows no sign of leveling off at large q.
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