ULTRASONIC RELAXATION IN MOLECULAR CRYSTALS

Abstract
One of the main mechanisms of ultrasonic attenuation in molecular gases and liquids is due to a slow transfer of energy between external and internal degrees of freedom of molecules. In 1959 Liebermann propounded the extension of this relaxation to the solid state and used the approach soon derived in gaseous and liquid phases. A Boltzmann-equation approach is suggested here which makes it possible to consider this ultrasonic relaxation as an anomaly of the Akhieser loss. Moreover it is shown that anisotropy of the solid state does not act on the frequency dependence of the relaxational term but mainly on its magnitude whose expression used up to now is proving incorrect ; a new expression is given.