We describe optical studies of dilute noble-metal-noble-metal alloys. A differential technique compared the reflectance of the alloy with the reflectance of the pure metal and measured a quantity proportional to just the difference in reflectance. The method was particularly sensitive to small changes in reflectances and could be used to study very dilute alloys-the alloys here were between $\frac{1}{10}$- and 3-at.% impurity concentration. The measurements indicated that the noble alloys are of two types. In copper-gold and silver-gold alloys the electron energy bands shift smoothly as the concentration varies. The rates and directions of shifts are determined not only by differences in the host and impurity potentials, but also by the changes in lattice constant, and we emphasize the importance of the latter. In contrast, in copper-silver alloys separate host and impurity $d$ bands are formed.

• Received 3 December 1971

DOI:https://doi.org/10.1103/PhysRevB.6.1209