Ion Excitation of Characteristic X Rays for Elements with72<~Z<~92

A systematic study has been made of the ionization cross sections for ion excitation of the $M$-shell x rays of elements with $Z$ in the region $72\le Z\le 92$. Protons, $\mathrm{H}_2^{}{}_{}{}^{+}$ ions, and $\alpha$ particles with incident energies ranging from 60 to 400 keV were used to excite the $M$ lines of Hf, Ta, W, Au, Bi, and U. The $Z$ dependence of the proton-excited x-ray yield for these elements, although differing sharply from previously published $M$-shell results, is shown to be consistent with the $Z$ dependence for proton excitation of the $K$ and $L$ shells reported in earlier papers. Finally, we have compared the ionization cross sections of $\mathrm{H}_2^{}{}_{}{}^{+}$ ions at incident energy $E_0$ with those for protons at incident energy $\frac{1}{2}{E}_0$, and found that within 30% they are in the ratio 2: 1.