Brillouin scattering studies in simple liquids, oxygen and nitrogen

Morgan, Ian Eric (1976) Brillouin scattering studies in simple liquids, oxygen and nitrogen. Masters thesis, Memorial University of Newfoundland.

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Abstract

Thermal Brillouin scattering techniques were used to determine the velocity and attenuation of hypersonic (0.4 to 5 GHz) waves in liquid oxygen and nitrogen. Each liquid was maintained under conditions of equilibrium with its vapour, and experiments were carried out over the temperature range from the normal boiling point (Tnbp) to within 1 K of the critical point (Tc) in each case (90.19 K to 154.58 K for liquid oxygen, and 77.36 K to 126.26 K for liquid nitrogen). -- Light from a frequency stabilized Ar⁺ laser was focused at the centre of a temperature controlled cell containing the liquid sample. Light scattered at 90ﾟ was then analyzed by a Fabry-Perot interferometer, the transmitted light being detected by a cooled photomultiplier tube. After phase-sensitive amplification, the resulting Brillouin spectrum was displayed on a chart recorder. -- The hypersonic velocity results obtained showed excellent agreement with the corresponding ultrasonic (1.2 MHz) velocities obtained from the literature for both liquids except for temperatures near the respective critical points. In this region the hypersonic velocities obtained for both liquids were lower in magnitude than the corresponding ultrasonic velocities. For oxygen (Tc = 154.58 K) this discrepancy increased from 4% at 150 K to 13% at 153.9 K, while for nitrogen (Tc = 126.26 K) this discrepancy ranged from ∿ 1% at 123.7 K to ∿ 3% at 125.5 K. Since these discrepancies are significantly greater than the estimated errors in these regions (a maximum of ± 1% for the hypersonic velocities, ± 0.05% for the ultrasonic velocities), it is concluded that these saturated liquids exhibit significant negative dispersion in the sound velocities at temperatures immediately below the critical point. -- Measurements of the acoustic absorption coefficient were also made in these same temperature ranges. The results obtained for both of these liquids behaved similarly. Away from the critical point (i.e., ∣T - Tc∣ > 10 K), the absorption coefficient was found to increase gradually with temperature such that the ration of the absorption coefficient to the square of the hypersonic frequency varied approximately linearly with temperature. However, as the critical region was approached further (∣T - Tc∣ ≤ 10 K), the absorption coefficient increased rapidly by approximately one order of magnitude. Such a behaviour appears to be characteristic of these critical regions.

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