On the Vacuum-Ultraviolet Reflectance of Evaporated Aluminum before and during Oxidation*

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Abstract
The vacuum-ultraviolet reflectance of freshly evaporated aluminum films has been measured as a function of time in vacuum after the deposition. The pressure in the experimental chamber during the measurements ranged from 4 × 10−6 mm Hg at the time of the deposition to an ultimate pressure of 2 × 10−7 mm Hg. The first measurements were made less than 10 sec after the evaporation. The reflectance of unoxidized aluminum has been determined by extrapolating the reflectance curves back to zero time. It has been found that unoxidized aluminum has a normal-incidence reflectance greater than 86% for wavelengths down to λ1025 Å. The effect on the reflectance of exposing freshly prepared aluminum films to O2, N2, and air is shown. At λ735 and λ584 Å, the optical constants of unoxidized aluminum have been determined from measurements of reflectance as a function of incidence angle. The optical constants are: at λ735 Å, n = 0.455, k = 0.043; and at λ584 Å, n = 0.71, k = 0.018. The effect of oxidation on the reflectance of aluminum at λ735 and λ584 Å is shown, and the transmission of unsupported films of aluminum, with and without oxide, is calculated.