Why the Brillouin Light Scattering Relaxation Times of Molten Zinc Chloride Are Shorter and Weaker in Temperature Dependence than the Structural Relaxation Times from Depolarized Light and Neutron Spin Echo Spectroscopy.

A longstanding problem in the Brillouin light scattering (BLS) study of polymers is the relaxation times τ() being more than an order of magnitude shorter than the α-relaxation times τ() determined by dielectric, depolarized light scattering (DLS), and molecular dynamics simulations. In tackling the problem, τ() was identified with the relaxation time τ() of the primitive relaxation in the coupling model, which can be calculated from τ() and the stretch exponent β of the Kohlrausch correlation function for the α-relaxation.. The problem was solved by finding that indeed τ() is in good agreement with τ(). A recent work performed the neutron spin echo study of the structural α-relaxation of the network ionic liquid ZnCl and found the same anomaly as polymers. The α-relaxation time τ() from neutron spin echo (NSE) as well as the α-relaxation time τ() from DLS of ZnCl are much longer than τ() from BLS obtained before by several research groups. The finding of the same anomaly in ZnCl and polymers with very different chemical and physical structures offers an opportunity to critically test the explanation given before. The test was carried out by calculating the primitive relaxation times τ() and τ() from τ() and τ(), respectively, in zinc chloride. Good agreements of τ() from BLS with τ() and τ() were found and thus the explanation given for polymers remains valid for ZnCl. The test was extended to glycerol by comparing τ() with τ() and τ() calculated from the α-relaxation time τ() and τ() from incoherent and coherent neutron scattering, respectively. There is good agreement between τ() and τ() in glycerol. There is also semiquantitative agreement of τ() with τ() from dielectric spectroscopy as well as τ(). Thus, the explanation for polymers is verified in the two very different glass formers, ZnCl and glycerol, and it is an advancement in the application of BLS to study the dynamics of glass formers.