Increasing the Far-Ultraviolet Reflectance of Silicon-Oxide-Protected Aluminum Mirrors by Ultraviolet Irradiation

Abstract

The effect of ultraviolet irradiation on the visible and ultraviolet reflectance of silicon-oxide-protected aluminum mirrors was studied. The silicon oxide protective films were produced by evaporating silicon monoxide under various evaporation conditions. High deposition rates at low pressure result in films of true SiO which are absorbing in the ultraviolet and shorter wavelength region of the visible, and which are, therefore, unsuitable for protecting mirrors that are to be used in this spectral region. Films deposited at low rates and rather high pressures of oxygen (deposition rate <5 Å/sec at 8-10× 10-5 Torr) consist predominantly of Si2O3, and exhibit strong absorptance only below λ=3000 Å. Their ultraviolet absorptance can be completely eliminated by ultraviolet irradiation from a quartz mercury burner. This makes it possible to produce well-protected aluminum mirrors with 92% reflectance down to λ= 2200 Å. Infrared measurements have shown that the elimination of the ultraviolet absorptance of Si2O3 films by ultraviolet treatment is not caused by a conversion of Si2O3 into Si2O3. The reflectance of aluminum coated with true SiO is not affected by ultraviolet irradiation. Data on the solar absorptance of silicon-oxide-protected aluminum mirrors are presented.