Collective modes in molten alkaline-earth chlorides. I. Light scattering

Abstract

Light scattering spectra have been measured for molten MgCl₂, CaCl₂, SrCl₂ and BaCl₂. The temperature dependence of the spectra has been studied for MgCl₂ and CaCl₂. Anhydrous samples were prepared from hydrated salts by dehydration under high vacuum (CaCl₂ and BaCl₂), by dehydration in the presence of NH₄Cl followed by sublimation (MgCl₂) and by dehydration and zone refining (SrCl₂). The spectra, while of the same general form as those found for the alkali halides, show one marked difference in that MgCl₂ exhibits a well resolved peak in the polarised spectrum. The spectra of CaCl₂, SrCl₂ and BaCl₂ show incompletely resolved shoulders which are more pronounced than those found in the spectra of the molten alkali iodides. A new method of analysis has been applied to the data from the molten alkaline-earth chlorides. Fourier transforms have been computed and the spectra have been examined in the time domain. The subsequent analysis is based on the hypothesis that the spectra are associated with the correlation functions of the charge-rather than the mass-currents. Thus there is little or no influence from Rayleigh-Brillouin scattering but it is anticipated that there will be a three-line spectrum associated with the charge currents. These are shown to be the equivalent of the first-order Raman effect while the second-order Raman effect gives rise to high-frequency exponential tails.