Optical Constants of Fluorite in the Extreme Ultraviolet

The reflecting power of a polished fluorite surface has been measured at nineteen wave-lengths from 1605A to 927A for 45^∘{}, 60^∘{}, 75^∘{} and 85^∘{} incidence, by means of a vacuum spectrograph. The method of photographic photometry followed is discussed in detail. The reciprocity law was employed and correction made for its failure in accordance with the work of Harrison and Leighton on oiled photographic films. Correction exposures were made with an inverse square law sensitometer. A vacuum chamber was arranged for this sensitometer since it was found that the reciprocity characteristics of the film were changed when it was placed in vacuum. The reflecting power data above λ1230A shows that the light dispersed by the grating was very nearly unpolarized.The curves for index of refraction and extinction coefficient have been computed from the reflecting power data for fluorite. The extinction curve shows an absorption band of roughly 100A half-width, with maximum near λ1115A. The dispersion is anomalous in this region. The absorption increases again below λ1060A. The condition of the fluorite surface is discussed. Evidence is given indicating the presence of a surface layer having a higher absorption coefficient and reflecting power than fluorite itself.