Interaction-induced spectra of molten alkali-metal and alkaline-earth halides

Abstract

In recent years accurate measurements have been made of the Raman spectra (1 < /cm^–1 < 500) of a large number of molten alkali-metal and alkaline-earth halides. These spectra have the following characteristics: (a) an order-of-magnitude increase in (integrated) intensity from the corresponding crystal spectra; (b) a narrow (h.w.h.h. ≈ 15 cm^–1) central peak, (c) an exponential tail at high , (d) a non-central (100 < /cm^–1 < 300) shoulder or peak and (e) a wide variation in depolarisation ratio from ca. 0.15 for Na⁺ salts to ca. 0.6 for Cs⁺ salts. All of these features can be explained as being interaction-induced. The interactions range from binary collisions to collective (‘quasi-molecular’ or ‘quasi-phonon’) modes. In the case of alkaline-earth halides, where the non-central contribution is better defined than for the alkali-metal halides, a semi-empirical model has been developed that enables a determination of the relaxation times corresponding to the various processes to be made.