Charge-current contribution to low-frequency Raman scattering from glass-forming ionic liquids

A low-frequency Raman study on the fragile ionic glass-forming system 2BiCl₃-KCl is reported. Approaches that are usually followed for a quantitative interpretation of the frequency and temperature dependence of the spectra have been employed. The inadequacy of satisfactorily fitting the experimental data applying standard models has led to the introduction of an extra contribution having its origin in scattering caused by charge-current fluctuations. The frequency dependence of the depolarization ratio, which is usually overlooked in low-frequency Raman studies, is another fact that reveals the necessity of the approach followed. We put forward a partial depolarization ratio analysis of the individual contributions involved in the low-frequency Raman spectrum. The characteristic frequency dependence of the depolarization ratio that each component exhibits is discussed alongside the corresponding physical mechanism on which the particular scattering contribution originates. Although applicable only to ionic glass-forming systems this picture stems from a well-grounded physical basis and may aid towards a better comprehension to selected aspects for supercooled liquids.