The Transparency of Sodium Fluoride and Lithium Fluoride in the Extreme Ultraviolet

Artifically prepared single crystals of sodium and lithium fluorides were tested for transparency to extreme ultraviolet light by means of a one meter concave grating vacuum spectrograph. The source used was a highly condensed spark discharge through a Pyrex capillary. These crystals were expected to be more transparent than calcium fluoride (fluorite) due to their lower indices of refraction. Lithium fluoride was transparent to 1083A (132A below the limit of fluorite) which is the practical limit to be expected. Sodium fluoride was transparent to 1320A, which is not thought to be the limit of the pure material. Crystals of these salts have been made as much as 5 cm in diameter, and they are rather insoluble and easily cut and polished being very similar to fluorite.