More or less uniform cells of ultraviolet transparent fused quartz about 1 µ thick and over 2 in. in diam have been produced and filled with sodium-potassium alloy, which is liquid at room temperature. These filters, while opaque in the visible, are transparent in the ultraviolet, with a cutoff wavelength lying between that for pure sodium (2100 A) and pure potassium (3400 A) and determined by the alloy composition. A tentative formula for the calculation of the cutoff wavelength is proposed and tested. The optical constants for the alloy were determined and are compared with those of the pure metals and with Kronig’s theory.
C. H. SHAW and W. T. FOREMAN, "Ultraviolet Transparent Alkali Metal Filters," J. Opt. Soc. Am. 49, 724-727 (1959)