Infra-red Dispersion Wave-lengths for Solid Lithium Fluoride containing Varying Proportions of Lithium Isotopes

Abstract

IN a programme utilizing isotopic mass as a probe for the solid state, the infra-red dispersion wave-lengths of ⁶Li¹⁹F and ⁷Li¹⁹F were recently obtained¹. Since then, improved apparatus and techniques have allowed a somewhat better estimate of these wavelengths. Moreover, the study has been extended to lithium fluoride containing varying proportions of the lithium isotopes. Lithium fluoride was prepared by the action of hydrofluoric acid on metallic lithium and its oxide, nitride, etc., resulting from exposure to the atmosphere. The enrichment of the lithium-6 sample was specified as 99.3 ± 0.2 per cent, and that of the lithium-7 as 99.8 ± 0.1 per cent. Powdered samples of the appropriate composition were evaporated from a platinum boat on to ¾-mil polyethylene sheet. The thickness of the film of lithium fluoride as measured by the a.c. conductivity of a known volume of distilled water in which a fiduciary portion of the film plus sheet had been steeped, was about 0.2µ. Infra-red spectra were obtained by use of a Perkin–Elmer ‘Model 21’ double-beam spectrophotometer with a cæsium bromide prism.