Reflectance of Evaporated Rhenium and Tungsten Films in the Vacuum Ultraviolet from 300 to 2000 Å*

Abstract

The reflectance of evaporated rhenium (Re) and tungsten (W) was measured in the wavelength region from 300 to 2000 Å. Re and W of 99.99% purity were evaporated with a 6-kW fine-focused electron gun and deposited on glass and fused-quartz plates of various temperatures ranging from 40 to 500°C. With Re, films of highest reflectance were obtained by evaporation onto unheated substrates, whereas with W, heating of the substrate greatly increased the reflectance of the deposited films. Both materials showed higher reflectances as rather thin, semitransparent films than as opaque ones. With coatings produced under optimum conditions, Re films had a reflectance maximum of 36% at 630 Å and tungsten films had two reflectance maxima, at 630 and 970 Å, of about 33% each. This is higher than that of any mirror coating presently used at these wavelengths. For both metals, the reflectance losses during extended exposure to air remained rather small, indicating that the oxide films formed on both film materials at room temperature were very thin. Re is a particularly useful coating material for replica gratings that cannot be heated during the deposition, because highest reflectance is obtained with unheated substrates.