One of the unsolved
problems of dynamics in supercooled liquids
are the differences in spectral shape of the structural relaxation
observed among different methods and substances, and a possible generic
line shape has long been debated. We show that the light scattering
spectra of very different systems, e.g., hydrogen bonding, van der
Waals liquids, and ionic systems, almost perfectly superimpose and
show a generic line shape of the structural relaxation, following
∝ ω–1/2 at high frequencies. In dielectric
spectra the generic behavior is recovered only for systems with low
dipole moment, while in strongly dipolar liquids additional cross-correlation
contributions mask the generic structural relaxation.
The slow Debye-like relaxation in the dielectric spectra of monohydroxy alcohols is a matter of long-standing debate. In the present Letter, we probe reorientational dynamics of 5-methyl-2-hexanol with dielectric spectroscopy and depolarized dynamic light scattering (DDLS) in the supercooled regime. While in a previous study of a primary alcohol no indication of the Debye peak in the DDLS spectra was found, we now for the first time report clear evidence of a Debye contribution in a monoalcohol in DDLS. A quantitative comparison between the dielectric and DDLS manifestation of the Debye peak reveals that while the dielectric Debye process represents fluctuations in the end-to-end vector dipole moment of the transient chains, its occurrence in DDLS shows a more local signature and is related to residual correlations that occur due to a slight anisotropy of the α relaxation caused by the chain formation.
We suggest a way to disentangle self- from cross-correlations in the dynamic susceptibility of the glass former glycerol by combining dielectric relaxation (BDS) with light scattering (DDLS), field cycling NMR, and shear compliance (SC) data.
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