Bremsstrahlung Resonances and Appearance-Potential Spectroscopy near the3dThresholds in Metallic Ba, La, and Ce

The high polarizability of the $3d$ shells in Ba, La, and Ce leads to a dynamically screened $E_g\to 4f$, $\hslash \omega$ electron-photon coupling, having pronounced resonances due to intermediate $3d^{9}4f^2$ levels. We qualitatively and even quantitatively reproduce the asymmetric bremsstrahlung intensity resonances experimentally observed by Liefeld, Burr, and Chamberlain.