Raman line shape analysis as a mean characterizing molecular glass-forming liquids

Abstract

Raman scattering spectra in glass forming toluene were studied in the temperature range 50–323 K with the goal of extracting information about homogeneous, inhomogeneous and orientational broadening. It was found that the temperature dependence of inhomogeneous line width allows one to depict two peculiar temperatures: T A and T g, where T g is the glass transition temperature and T A is the temperature of transition from an Arrhenius-like to a non-Arrhenius behavior for the α-relaxation time dependence on temperature, τ α( T). Temperature dependence of the orientational phase loss time τ OPL was found to correspond well to τ α at T > T A and continues approximately Arrhenius behavior for lower temperature in contrast to τ α( T). Also, a comparative analysis of homogeneous broadening of polarized and depolarized lines was done, which provided an estimation of the orientational broadening γ NL( T). The found γ NL( T) decreases linearly as the temperature decreases and goes to zero at T ~ T c, where T c is the critical temperature in framework of the mode-coupling theory (note that T c is close to other peculiar temperatures T B and T β, but we did not intent to distinguish among them in the present work). Thus, it was shown that the Raman line shape analysis in molecular glass forming materials allows one to extract peculiar temperatures: T A, T g, and, probably, T c. The test of the possibility to use a probe molecule for the Raman line shape analysis has revealed that the extracted data for probe molecule lines do not characterize the host matrices, at least in the low-viscous state ( T > T A).