Thermo-optic behaviour of solids. V. Alkali halides

Abstract

A direct verification of the general theory of thermo-optic behaviour has been obtained by applying it to the alkali halides. Rocksalt, sylvine and potassium iodide have been considered in detail. For this purpose, new dispersion formulæ embodying observed absorption frequencies have been developed for NaCl and KI. With KI, it is found that the proportionate variation χ [=-d (log v)/dt] of the first ultra-violet frequency at 2190 Å.U. measured by Fesefeldt agrees with what is calculated from the dn/dt data, thus verifying the fundamental basis of the author's theory. The theory successfully explains the whole course of the variation of dn/dt with wavelength in the case of rocksalt, and in particular the positive values of dn/dt near about 2000 Å.U. and its reversal in sign with increase of wavelength. The first three ultra-violet frequencies have χ's of the order of +100×10^-6, +25×10^-6 and -30×10^-6 respectively, the last one near 500 Å.U. not varying with temperature. With sylvine,dn/dt measurements in the visible and near infra-red are satisfactorily explained with similar values of χ as in rocksalt. The paper also contains a general review of the dispersion data for the alkali halides. It is shown that a dispersion formula of the Drude form is more appropriate for these salts than one of the Lorentz-Lorenz form.