Photoabsorption near the $L_{\mathrm{I}\mathrm{I},\mathrm{I}\mathrm{I}\mathrm{I}}$ Edge of Silicon and Aluminum

The absorption coefficients of Al, Si, and SiO were determined in the extreme uv with high spectral resolution, using synchrotron radiation. A detailed comparison of the $L_{\mathrm{I}\mathrm{I},\mathrm{I}\mathrm{I}\mathrm{I}}$ edge in the metal and in the semiconductor shows important differences. The spikes observed at the edge in Al are at least consistent with the predictions of recent theory. The silicon $L$ threshold shows the expected spin-orbit splitting of the initial states, but the shape just beyond the edge is not in good agreement with the results of one-electron band theory. The $L_I$ edges are located at 117.4 eV in Al and at 151 eV in Si. Great similarities are noted in the broad structure, 15 or more eV beyond the $L_{\mathrm{I}\mathrm{I},\mathrm{I}\mathrm{I}\mathrm{I}}$ edges in the three materials studied and also in Na and Mg films. It is suggested that electron-hole transitions accompanied by collective excitations are involved.