TheM-Series Absorption Spectrum of Metallic Bismuth

With a vacuum spectrometer and sputtered bismuth films as absorbers, the five $M$-absorption edges were obtained. The large discrepancies between computed and observed wave-lengths of Bi $M_{\mathrm{IV}}$ and $M_{\mathrm{V}}$ were verified. The $M_{\mathrm{I}}$, $M_{\mathrm{II}}$ and $M_{\mathrm{III}}$ discrepancies were found to be of the order of experimental error. To explain the $M_{\mathrm{IV}}$ and $M_{\mathrm{V}}$ discrepancies it is assumed that electrons from $M_{\mathrm{IV}}$ and $M_{\mathrm{V}}$ levels, in an absorption act, go to virtual lattice levels above the filled conduction, or valence, levels of the crystal for the elements Ta73 to Th90. Data on, and Kronig's theory of, secondary x-ray absorption are shown to support this view. The application of the ordinary x-ray selection rule $\Delta l=\pm 1$, $\Delta j=\pm 1$ or 0 in $M$ absorption, suggested by Siegbahn, is further discussed. That there is a definite selection rule operating in $M$ absorption there is little room to doubt.