Rayleigh-Brillouin spectroscopy of simple viscoelastic liquids.

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.