Since the discovery that Al overcoated with MgF2 or LiF produces high reflectances to wavelengths as short as 1150 Å and 1000 Å, respectively, these coatings have been used extensively in vacuum ultraviolet instruments in the wavelength region where their reflectance is high. If the instrument is intended to cover wavelengths shorter than the two given above, usually either Pt or Ir is used, with a loss of speed at the longer wavelengths. This paper presents reflectance data showing that fluoride-overcoated Al can be useful to wavelengths as short as 500 Å. Measurements were made from 1600 Å to about 300 Å at normal, 35°, and 85° angles of incidence, angles used in normal, Seya, and grazing incidence spectrometers, respectively. These measurements show that from the boundary of the high reflectance region to 500 Å, the reflectance at normal and 35° depends on the thickness of the fluoride coating and can be as high as 24% at 800 Å for a MgF2 thickness of 150 Å. For shorter wavelengths, the reflectance shows a decreasing thickness dependence and at 304 Å is very low—about 1%. At grazing incidence, the reflectance shows some thickness dependence from 1500 Å to about 1000 Å, but toward shorter wavelengths the dependence disappears and the reflectance increases slowly to about 80% at 500 Å. In addition to the reflectance measurements, polarization effects are discussed.
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