Comment on structure near the cutoff of the continuous x-ray spectrum of lanthanum

Recently, Liefeld, Burr, and Chamberlain observed a structure just below the cutoff frequency of the continuous x-ray spectrum of lanthanum. In an attempt to understand the origin of this feature, we have numerically calculated the tip-region electron-lanthanum atomic bremsstrahlung spectrum, obtained in an independent-electron approximation with the Hartree-Slater potential, for incident-electron kinetic energies from 400 to 1500 eV. The calculations dogive a structure qualitatively similar to the observation. The enhancement occurs in the $f$-wave channel for the final scattered electron, which exhibits a resonance. Such a resonance may be expected because at this point in the periodic table the $4f$ subshell is about to start filling. A more quantitative description would require inclusion of many-electron and solid-state effects.