MIVandMVAbsorption Lines and Edges ofGd64,Dy66, andYb70

The $M_{\mathrm{IV}}$ and $M_{\mathrm{V}}$ edges of ${}_{64}{}^{}{}_{}{}^{}\mathrm{Gd}$, ${}_{66}{}^{}{}_{}{}^{}\mathrm{Dy}$, and ${}_{70}{}^{}{}_{}{}^{}\mathrm{Yb}$ were measured using a high-intensity-focusing x-ray spectrograph operating in vacuum. An absorption line is detected on the long-wavelength side of the $M_{\mathrm{IV}}$ edge of ${}_{64}{}^{}{}_{}{}^{}\mathrm{Gd}$ and a weaker one at the $M_{\mathrm{V}}$ edge of ${}_{66}{}^{}{}_{}{}^{}\mathrm{Dy}$. For ${}_{70}{}^{}{}_{}{}^{}\mathrm{Yb}$, however, the $M_{\mathrm{V}}$ absorption has the appearance of a strong "white line." No absorption line has been observed at the $M_{\mathrm{V}}$ edge of ${}_{64}{}^{}{}_{}{}^{}\mathrm{Gd}$ nor at the $M_{\mathrm{IV}}$ edges of ${}_{66}{}^{}{}_{}{}^{}\mathrm{Dy}$ and ${}_{70}{}^{}{}_{}{}^{}\mathrm{Yb}$. The significance of the occurrence and absence of absorption lines in the filling of the $4f$ shell of the "heavy" rare earths and the effect of the absorber thickness on the shape of the absorption profile are discussed.