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 com puter simulation directly from molecular dynamics and by using an instantan eous normal modes (INM) approach. Good agreement between the spectra is rep orted. The calculations use a realistic model for the fluctuating polarizab ility of ZnCl2, derived from earlier work on simpler ionic melts. This cont ains several mechanisms which couple the radiation field to the ionic motio n-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 effi ciency of the coupling of a given mode to the radiation field depends stron gly on its character and on the polarizability mechanism, so that the Raman spectra predicted for the different mechanisms differ markedly. A conseque nce is that the discrete Raman bands observed at high frequency in the pola rized spectrum do not coincide with the spectrum of the localized, quasi-mo lecular ZnCl4 units of the network. Furthermore, the "light-vibration'' cou pling, relating the reduced Raman spectrum to the underlying vibrational de nsity of states, is appreciably frequency dependent and different for each mechanism. (C) 1999 American Institute of Physics. [S0021-9606(99)50110-4].