Raman scattering in the network liquid ZnCl2 relationship to the vibrational density of states

The light scattering (Raman) spectrum of ZnCl2 has been calculated in a computer simulation directly from molecular dynamics and by using an instantaneous normal modes (INM) approach. Good agreement between the spectra is reported. The calculations use a realistic model for the fluctuating polarizability of ZnCl2, derived from earlier work on simpler ionic melts. This contains several mechanisms which couple the radiation field to the ionic motion—short-range, dipole-induced dipole and hyperpolarization. INM analysis of ZnCl2 has previously shown how the character of the underlying vibrational modes changes across the density of states. Here it is shown that the efficiency of the coupling of a given mode to the radiation field depends strongly on its character and on the polarizability mechanism, so that the Raman spectra predicted for the different mechanisms differ markedly. A consequence is that the discrete Raman bands observed at high frequency in the polarized spectrum do not coincide with the spectrum of the localized, quasi-molecular ZnCl4 units of the network. Furthermore, the “light-vibration” coupling, relating the reduced Raman spectrum to the underlying vibrational density of states, is appreciably frequency dependent and different for each mechanism.