Reflectance Studies of Ba, Sr, Eu, and Yb

Abstract

In situ reflectance measurements have been made on films of the alkaline-earth metals barium (bcc) and strontium (fcc), and rare-earth metals europium (bcc) and ytterbium (fcc) from 1.0 to 11.6 eV. Extreme care was taken in all aspects of the work so that a Kramers-Kr\"onig analysis of the reflectance data yielded optical constants ${\ensuremath{\epsilon}}_{1}$, ${\ensuremath{\epsilon}}_{2}$, $\ensuremath{\sigma}$, and $\ensuremath{\alpha}$ which were accurate to better than \ifmmode\pm\else\textpm\fi{} 20%. The volume-loss functions calculated from these optical constants yielded an agreement with loss-function peak positions obtained in Kunz's high-energy electron-loss experiments which was not seen in previous optical studies, and served as verification of the improved accuracy of the present study. A comparison of the interband optical conductivities for the alkaline-earth and rare-earth metals failed to indicate strong structure or increased oscillator strength in the rare-earth metals which could be unambiguously attributed to transitions from $4f$ electrons found in europium and ytterbium. The filled $4f$-electron states in these two metals are known to be accessible to the spectral range of the present study, and an explanation, based on matrix elements, is proposed for this inability to excite such electrons. The structure which was observed in the interband conductivity of all four of these metals was correlated to crystal structure and showed much better agreement with the predictions of strong $d$-electron effects implied in recent band calculations than it did with the simple predictions of the nearly-free-electron theory.