The energy-loss distributions of ≈20-keV electrons transmitted through 210-575 Å-thick specimens of Al and ${\mathrm{Al}}_2$${\mathrm{O}}_3$ have been measured at zero scattering angle. Two types of ${\mathrm{Al}}_2$${\mathrm{O}}_3$ were used: films of the $\gamma$ phase and amorphous films prepared by anodization. Loss spectra were obtained for an energy-loss range of 0-160 eV, in which structure due to $L$-shell excitation in Al and ${\mathrm{Al}}_2$${\mathrm{O}}_3$ was observed. The positions of the absorption edges and of the peaks on the high-energy-loss side of each edge agreed well with recent x-ray absorption measurements. Slightly different structure for the two forms of ${\mathrm{Al}}_2$${\mathrm{O}}_3$ was found near the edge, as recently found by x-ray absorption. From the observed intensities, the values of oscillator strengths, mass-absorption coefficients, and electron-scattering cross sections have been derived for each material up to ≈85 eV beyond the absorption edge. The Al results are in good agreement with recent x-ray measurements and are consistent with the ${\mathrm{Al}}_2$${\mathrm{O}}_3$ data. The similarity in the electron energy-loss and x-ray absorption measurements is consistent with the equality of the longitudinal and transverse dielectric constants $\epsilon \left(\omega \right)$ and with the expectation that in this energy-loss region $\mathrm{Im}\epsilon =-\frac{\mathrm{Im}1}{\epsilon }$ to a good approximation.

• Received 5 July 1967

DOI:https://doi.org/10.1103/PhysRev.167.592