Experimental and Theoretical Study of the Absorption of Ultrasoft X Rays in Some Heavy Elements

The orbital radiation of the Frascati synchrotron has been used to determine the mass absorption coefficients of some heavy elements in the 20 and 130 Å range. Results have been obtained for tantalum, platinum, gold, and bismuth. The photoionization cross sections of the $4d$, $4f$, and $5p$ electron shells have been calculated from a free nonhydrogenic atomic model. A comparison of the experimental and theoretical results shows that the photoionization in the $4f$ and $5d$ shells plays a fundamental role, and that the combined effects of these shells explain the variations of the absorption coefficient in the range examined. Moreover, certain details in the spectra tend to indicate the existence of multiple excitation mechanisms.