J. Wiedersich

A comprehensive light scattering study of the glass former toluene

We present depolarized light scattering data of the glass former toluene as obtained from tandem Fabry–Perot interferometry and Raman scattering covering a frequency range 0.5 GHz<v<5000 GHz. A large temperature interval of the (supercooled) liquid (Tg=117 K<T<295 K) as well as of the glass (7 K<T<117 K) is measured. Testing the scaling laws of idealized mode coupling theory (MCT) we find that MCT provides a satisfying interpolation up to the fluid regime (T/Tg=2.5) and a critical temperature Tc≅153 K is extracted. Deviations from the high-temperature MCT laws below Tc can be identified by constructing a master curve for the α-process. The deviations are attributed to spectral contributions from the high-frequency wing of the α-process and/or of the slow β-process which appear apparently only below Tc. A comparison with dielectric spectroscopy data, exhibiting a strong β-process, confirms this. Furthermore, we carry out a phenomenological analysis which assumes additivity of the susceptibility of α-proces...

Spectral shape of relaxations in silica glass

Precise low-frequency light scattering experiments on silica glass are presented, covering a broad temperature and frequency range ( 9 GHz<nu<2 THz). The spectra show a power-law low-frequency wing of the relaxational part of the spectrum with an exponent alpha proportional to temperature in the range 30<T<200 K. A comparison of our results with those from acoustic attenuation experiments performed at different frequencies shows that this power-law behavior rather well describes relaxations in silica over 9 orders of magnitude in frequency. These findings can be explained by a model of thermally activated transitions in double well potentials.

Fast and slow relaxation processes in glasses

We present dielectric relaxation (DS) and light scattering (LS) data of several glass formers. Relaxational features are compiled which are not yet properly taken into account by current models. (i) We distinguish two types of glass formers. Type A systems do not show a slow -process whereas type B systems do. A full line-shape analysis of is presented . In type A systems the evolution of the high-frequency wing of the -process is the most prominent spectral change while cooling and leads to an essentially constant loss at . The analysis of of type B systems is carried out within the Williams-Watts approach and we focus on the temperature dependence of the -relaxation strength. (ii) Concerning fast relaxations below as revealed by LS we identify relaxation with a low-frequency power-law behaviour. No indication of a crossover to a white noise spectrum as previously reported and discussed within MCT is found. Analysing this relaxation we recourse to the model of thermally activated transitions in asymmetric double well potentials. We show that the model works well in some cases and the distribution of barrier heights may be extracted, but in other systems pronounced deviations occur.

Temperature and pressure dependence of protein stability: The engineered fluorescein-binding lipocalin FluA shows an elliptic phase diagram

We have measured the equilibrium constant for the denaturation transition of the engineered fluorescein-binding lipocalin FluA as a function of pressure and temperature, taking advantage of the fact that the ligands fluorescence is almost fully quenched when complexed with the folded protein, but reversibly reappears on denaturation. From the equilibrium constant as a function of pressure and temperature all of the involved thermodynamic parameters of protein folding, in particular the changes in entropy and volume, compressibility, thermal expansion, and specific heat, were deduced in a global fitting procedure. Assuming that these parameters are independent of temperature and pressure, we can demonstrate from the ratio of Δβ, Δα2, ΔCp that the phase diagram of protein folding assumes an elliptic shape. Furthermore, we can show that the thermodynamic condition for such an elliptic phase diagram is related to the degree of correlation between the fluctuations of the changes in volume and enthalpy at the phase boundary. For the protein investigated this correlation is low, as generally expected for highly degenerate systems. Our study suggests that the elliptic phase diagram is a consequence of the inherent conformational disorder of proteins and that it may be viewed as the thermodynamic manifestation of the high degeneracy of conformational energies that is characteristic for this class of macromolecules.

Fast relaxation in the structural glass and glassy crystal of ethanol and cyano cyclohexane: A quasielastic light scattering study

Fast relaxation processes in the structural glass and glassy crystal of ethanol as well as in the glassy crystal of cyano cyclohexane were studied by quasielastic light scattering (LS). Depolarized and polarized LS spectra together with the Brillouin line were measured applying a tandem Fabry–Perot interferometer at frequencies 1–1500 GHz. Depolarized, polarized LS and internal friction data exhibit very similar behavior, thus reflect the same relaxations. The depolarized LS spectra are described by assuming a nearly constant loss (NCL) contribution dominating in the frequency range from 1 GHz to a few tens GHz and by a fast dynamics contribution at higher frequencies. The NCL amplitude increases exponentially with temperature in a similar way as found by dielectric spectroscopy for other molecular glasses. The data cannot be described by thermally activated dynamics in double well potentials, a result contradicting the findings in inorganic glasses such as silica.

Light scattering spectra of fast relaxation in B2O3 glass

Light scattering spectra of B2O3 glass were measured in the frequency range 20–7000 GHz at temperatures in the range of 15–300 K and are compared with those previously obtained for Ca0.4K0.6(NO3)1.4 (CKN). The low-frequency contribution of the fast relaxation susceptibility spectrum is found to show a power-law behavior with an exponent α=0.6, the latter being temperature independent in contrast to the finding in other inorganic glasses such as silica and CKN. Possible explanations of the spectral shape in B2O3 are considered.

Inelastic light scattering in B2O3 glasses with different thermal histories

This contribution presents a systematic light scattering study of a series of boron oxide glasses which are characterized by different thermal histories. The thermal treatment was obtained by annealing the samples close to the glass transition temperature for times of several hours. Both low-frequency (0.1–30 cm−1) and high-frequency (5–1600 cm−1) spectra were monitored by using a tandem Fabry–Perot interferometer and a Raman spectrometer, respectively. The low-frequency spectra include quasielastic contributions and the boson peak. It was found that different thermal histories lead to pronounced changes in the low-frequency spectrum. The position of the boson peak shifts to higher frequencies and the magnitude of the quasielastic contribution decreases as a function of annealing time. Both quantities correlate linearly with the density of the samples (ρ=1.804–1.866 g/cm3). On the other hand, the high-frequency modes do not show discernible changes. In particular, no alteration of the modes which correspo...

Q DEPENDENCE OF LOW-FREQUENCY RAMAN SCATTERING IN SILICA GLASS

Accurate measurements of the dependence of low-frequency Raman scattering on the scattering angle were performed in two silica glasses. By a comparison of spectra measured at a large scattering angle (close to back scattering) and a small one (close to forward scattering), we for the first time observed a q-dependence of the low-frequency light scattering in glasses. From the magnitude of the effect, the vibration correlation length is estimated and compared with results from the picosecond optical technique; a reasonable agreement is found.