Optical constants of some silver alloys

The optical constants between 1.40 and 5.25 eV of three disordered alloy systems, AgCd, AgMg, and AgSn, have been directly measured over their respective $\ensuremath{\alpha}$-phase ranges. The measurements indicate that the onset of the ${{L}_{2}}^{\ensuremath{'}}\ensuremath{-}{L}_{1}$ transitions in pure Ag occurs at 3.33 \ifmmode\pm\else\textpm\fi{} 0.08 eV. The measurements clearly show the delineation of the alloys into two classes, one called class $A$, where the formalism of periodic structures with direct optical transitions is applicable, and one called class $B$, where this formalism is not applicable, and indirect optical transitions dominate in the changes caused by alloying. The plasma frequency in the Drude region is found to be a constant for these alloys within experimental error. For both of the class-$A$ alloys, AgMg and AgCd, the electronic structure is very similar and a non-rigid-band variation of energy levels is found which can be qualitatively understood in terms of alloy theory. AgSn is a class-$B$ alloy, the understanding of which is more difficult.