Recent theoretical work has suggested that at high frequencies, there should be significant departure from classical hydrodynamic behavior in simple fluids. In particular, the frequency dependence of transport coefficients is no longer negligible and may introduce observable effects into the propagation of high-frequency sound. We have measured the sound velocity of high-frequency phonons (1-3 GHz) in liquid argon and liquid neon along their vaporpressure equilibrium curves using the Brillouin scattering technique. The Brillouin spectra were excited with a single-mode argon-ion laser operating at 5145 or 4765 Å and were analyzed and detected with a Fabry-Perot interferometer and standard photoelectric techniques. Hypersonic (∼3 GHz) velocities observed in argon decrease linearly from 850 m/sec at 85 °K to 742 m/sec at 100°K and uniformly exhibit a small departure from low-frequency (1 MHz) data obtained under the same thermodynamic conditions. This effect is in qualitative agreement with theoretical-model predictions of a negative velocity dispersion at high frequencies. Our measurements of the sound velocity in liquid neon are the first in this material by any technique, and hance cannot be compared with ultrasonic values. The hypersonic velocity in neon decreases not quite linearly from 620 m/sec at 24.9°K to 508 m/sec at 32°K. When compared with results in other noble-gas liquids through corresponding-state arguments, these data suggest the existence of measurable quantum effects in the hypersonic velocity of liquid neon. In addition, an interesting change in slope of the velocity-versus-temperature curve (of 17%) is observed at 28°K.

• Received 23 May 1969

DOI:https://doi.org/10.1103/PhysRev.186.244