Reflectivity measurements on dilute gold-iron alloys by a new method are reported. There is clear evidence that the band gaps change steadily on alloying. In the region 3 to 6 eV additional absorption growing nonlinearly with concentration suggests that the impurities are interacting with each other.

• Received 1 November 1968

DOI:https://doi.org/10.1103/PhysRevLett.22.133