Absorption of ultrasound near the critical point of sodium-ammonia solutions

Abstract

Comprehensive measurements of ultrasonic absorption in sodium-ammonia solutions near the critical mixing point and passing through the metal-nonmetal transition are presented and analyzed in detail. A few ultrasonic-velocity measurements in the same system are also reported. The observed critical divergence of the absorption (all the data being in the hydrodynamic regime) is shown to obey the Fixman-Kawasaki multiple-relaxation calculation by the adjustment of three fit parameters. The measured absorption and the characteristic frequency and amplitude parameters all appear to undergo a scaling crossover at a reduced temperature of about ${10}^{\ensuremath{-}2}$. The presence of large-amplitude fluctuations in the solutions near the critical point is inferred as well as the presence of a distribution of relaxations related to the diffusive decay of these fluctuations. The results are also discussed in terms of the microscopic inhomogeneities or clusters proposed by Cohen and Jortner for metal-ammonia solutions.