The index of refraction and the extinction coefficient of thin films of zinc sulfide and cadmium sulfide evaporated on quartz substrates were determined in the spectral range from the visible to 2100 A. The optical properties were calculated from reflectance and transmittance measurements made with a Beckman DK Spectrophotometer equipped to measure specular reflectance. The index of refraction and the extinction coefficient of zinc sulfide have maxima at 2300 A and 2150 A, respectively. There is a maximum of the extinction coefficient of cadmium sulfide at 2300 A. The refractive index of cadmium sulfide has a maximum at 2500 A and a subsidiary maximum at 5000 A. There is a region of anomalous dispersion from 4000 A to 5000 A and a contribution to the absorption which are probably associated with the excitation of valence electrons into the conduction band.
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