Collective modes in molten alkaline-earth bromides. Light scattering

Light scattering spectra in the energy-transfer range 4< nu <500 cm -1 have been measured for molten MgBr 2 , CaBr 2 , SrBr 2 and BaBr 2 . The spectra are similar to those found for the molten alkaline-earth chlorides. First-order Raman scattering gives a central peak, whose width is determined by dissipative effects such as electrical conduction, and a noncentral peak or shoulder whose frequency is of the order expected for TO modes in the corresponding crystals. In the case of MgBr 2 the peak is well resolved and in CaBr 2 it is almost resolved. Second-order Raman scattering gives an exponential tail at high frequencies. The spectra have been fitted with the semi-empirical model of Bunten and co-workers (1984) in order to derive the relaxation times associated with these contributions to the scattering. In Mg 2+ and Ca 2+ salts, where the noncentral contribution is completely polarised, its frequency scales linearly with reduced mass.