- The Dynamics of Molecular Glasses Studied by Light Scattering (2006)
- The present work is devoted to studying the dynamics in molecular glass formers applying the light-scattering (LS) technique, in particular tandem Fabry-Perot interferometry which allows to cover the frequency range from 0.3 GHz to 1000 GHz. Chapters 4 to 7 each present, in a self-contained way, different aspects of the dynamics, as summarized below. In Chapter 4, the results of the study of the molecular glass formers 2-picoline and m-tricresyl phosphate are presented. The LS spectra are analyzed in the frame of the Mode Coupling Theory (MCT). At high temperatures the evolution of the susceptibility minimum is well described by MCT. Below the critical temperature Tc, the asymptotic scaling laws of MCT fail due to the appearance of the excess wing of the alpha-process, which shows a universal evolution as a function of relaxation time, as was demonstrated by dielectric spectroscopy. A phenomenological approach, which allows to separate slow (alpha-process) and fast relaxation processes in the LS spectra is developed. Applying this approach, the temperature dependence of the non-ergodicity parameter f is obtained. The anomaly of f as well as a crossover to "white noise" of the fast dynamics spectra is found. In Chapter 5, the most extensive dielectric data of glycerol compiled by Lunkenheimer et al. [Contemp. Phys. 41, 15 (2000)] are reanalyzed. In contrast to the analysis of Lunkenheimer et al., the normalized susceptibility spectra, i.e, the dielectric loss data normalized by the static susceptibility, including the high temperature data, are analyzed. For this purpose a phenomenological approach, which describes the whole dielectric spectrum including the alpha-peak, its high frequency wing, and fast dynamics, is applied. The crossover temperature extracted from the phenomenological analysis and defined by the emergence of the high frequency wing upon cooling agrees well with the critical temperature extracted from the MCT analysis. The crossover temperature Tc=288 is significantly higher than reported before. Extracting the non-ergodicity parameter f, the characteristic anomaly similar to the one of 2-picoline and m-TCP discussed in Chapter 4, is found. In Chapter 6, the study of the fast relaxation below Tg in the molecular glasses 2-picoline, m-TCP, o-terphenyl (OTP), as well as in ethanol is presented. In addition to the boson peak, the depolarized LS spectra reveal quasi-elastic contributions that we attribute to i) the nearly constant loss (NCL) in the frequency range below 10 GHz and ii) a power law contribution with positive exponent alpha at higher frequencies. In the majority of glasses the latter may be attributed to thermally activated dynamics in asymmetric double well potentials (ADWP), as was previously found for the DLS spectra in silica. Following the Gilroy-Phillips model, the exponent alpha shows a master curve as a function of T /Vo for the various glasses where Vo specifies the width of the exponential distribution of barriers g(V), i.e., g(V) is propotional to the exp(-V/Vo). In Chapter 7, the investigation of the fast relaxation processes in the structural glass (T<Tg) and in the glassy crystal phase of ethanol, as well as in cyano cyclohexane, is presented. Depolarized and polarized LS spectra including the Brillouin lines were measured. It was found that depolarized, polarized LS and internal friction data exhibit fairly similar behavior, and thus reflect the same relaxations. The DLS spectra were described by assuming that the NCL contribution dominates below a few tens GHz, while the fast relaxational dynamics dominates at higher frequencies.