Abstract
The absorption spectra of D-sorbitol and a range of its concentrated aqueous solutions were studied by terahertz spectroscopy over the temperature interval of 80 K < T < 310 K. It is shown that the slow-down of molecules at around the glass transition temperature, Tg, dramatically influences the thermal dependence of the absorption at terahertz frequencies. Furthermore, two different absorption regimes are revealed below Tg: at temperatures well below Tg, the absorption resembles the coupling of terahertz radiation to the vibrational density of states (VDOS); above these temperatures, between 160 K and Tg, in the sample of pure sorbitol and the sample of a solution of 70 wt% sorbitol in water, another type of absorption is observed at terahertz frequencies. Several possibilities of the physical origin of this absorption are discussed and based on the experimental data this process is tentatively assigned to the Johari-Goldstein β-relaxation processes shifting to lower frequencies at temperatures below Tg leaving behind a spectrum largely dominated by losses into the VDOS.
Highlights
Supercooled liquids and glasses and their fascinating physical properties have been the subject of extensive studies for many decades
The experiments showed that absorption in this part of the spectrum can be described by a general power-law, which can be explained in terms of disorder-induced coupling of farinfrared radiation to a density of low frequency Debye modes, known as the vibrational density of states (VDOS)
The results revealed that, similar to what is observed for liquid crystals, the Optical heterodyne detected optical Kerr effect (OHD-OKE) signal in supercooled liquids follows a temperature-independent intermediate power law starting at picosecond timescales
Summary
Supercooled liquids and glasses and their fascinating physical properties have been the subject of extensive studies for many decades. The results revealed that, similar to what is observed for liquid crystals, the OHD-OKE signal in supercooled liquids follows a temperature-independent intermediate power law starting at picosecond timescales Based on this finding the authors hypothesised that, as for liquid crystals, the picosecond orientational dynamics of supercooled liquids is a result of a pseudonematic domain structure.[10] In addition to the OHDOKE experiments scattering techniques such as inelastic light
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