The Absorption of Ionic Crystals in the Ultraviolet

In order to extend the measurements of Hilsch and Pohl on the ultraviolet absorption bands of the alkali halides below 1600A, lithium fluoride has been used as a support for thin films of the salts. Selected lithium fluoride plates of one millimeter thickness when properly cleaned were consistently transparent to 1050A. The compound to be studied was distilled onto the lithium fluoride within a Thibaud spectrograph. A silver solder Carboloy hot spark and Ilford Q plates were used to make the measurements. Comparison of spectra transmitted by the lithium fluoride plates with and without absorbing films of approximately 0.05 micron thickness show, in addition to the bands reported by Hilsch and Pohl, a number of bands below 1600A whose widths vary between 0.3 and 1.3 electron volts. Observations made on some of these compounds by A. Smith, using a less direct method, agree very poorly with these measurements. A comparison of the observed bands with the computations by A. v. Hippel shows fairly complete agreement. Results on several other halides show only a series of sharp absorption edges with continuous absorption toward shorter wave-lengths.