The specular reflectance of evaporated aluminum films has been measured with high precision (±0.1%) from 0.55 to 32 μ. The measurements were made at normal incidence on fresh and aged films deposited on fuzed quartz substrates with rapid evaporation rates at pressures of about 1×10−5 Torr. The dependence of the reflectance on the evaporation conditions was also investigated, and it was found that the infrared reflectance of aluminum prepared under the usual high-vacuum conditions is reproducible and insensitive to the conditions of film preparation. Although the measured reflectance of freshly evaporated aluminum films differs by only 0.1% at 30 μ from the value calculated from the free-electron approximation assuming bulk parameters for aluminum, the variation of the measured reflectance with wavelength does not fit the theory in the infrared. It is felt that the validity of the free-electron approximation should be further tested by using films deposited in ultra-high vacuum.
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