Abstract
The Rayleigh–Brillouin spectrum of 2,4-dimethyl pentane, 2,4,6-trimethyl heptane, 3,5-dimethyl heptane, 2,2,4,4-tetramethyl pentane, and 2,2,4,4,6,6,8,8-octamethyl nonane was measured as a function of temperature from well above the Brillouin linewidth maximum down to the glass transition region. Brillouin splittings and linewidths were determined at each temperature. The results were analyzed quantitatively in terms of the theory of Rytov. The spectrum of the Mountain peak due to slowly relaxing density fluctuations was obtained over the entire temperature interval where it appears. The spectrum was very non-Lorentzian at all temperatures. Near the glass transition the dynamics of the fluctuations associated with the Mountain peak were studied with photon correlation spectroscopy. The relaxation functions were observed to be highly nonexponential. The average relaxation time changed very rapidly with temperature, as expected near the glass transition. In addition to the slowly relaxing part of the Mountain peak observed with photon correlation spectroscopy, high frequency components were observed directly in the frequency domain spectra. These rapidly relaxing density fluctuations account for the finite Brillouin linewidths observed near the glass transition. A very broad distribution of relaxation times must be taken into account to explain the entire Rayleigh–Brillouin spectrum.
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