TheM-Series Absorption Spectrum of Tantalum

The $M$-series absorption spectrum of tantalum (73) has been investigated, with a vacuum spectrograph of the Siegbahn type. The absorption screens were metallic tantalum, prepared by an evaporation process. Spectrograms were obtained for three absorption limits at 5.64A ($M_{\mathrm{III}}$), 6.773A ($M_{\mathrm{IV}}$), and 6.997A ($M_{\mathrm{V}}$). When the experimental $\frac{\nu }{R}$ values are compared to values calculated from $L$-absorption and emission data, the experimental values for $M_{\mathrm{IV}}$ and $M_{\mathrm{V}}$ show a displacement toward higher frequencies and the $M_{\mathrm{III}}$ limit a displacement toward lower frequencies. These discrepancies are in agreement with those found by several other investigators in the region from W(74) to Au(79). The $M_{\mathrm{III}}$ limit has the appearance of a white line, and $M_{\mathrm{IV}}$ and $M_{\mathrm{V}}$ are diffuse in character. The possibility of an explanation of these discrepancies on the basis of transitions to levels in the periphery of the atom is suggested.