Sound propagation and vibrational relaxation in molecular crystals

Abstract

The connection between sound wave propagation and vibrational relaxation in molecular crystals is discussed within the excitation collision operator formalism. An extra-attenuation in molecular crystals with regard to usual dielectric crystals is shown to result from the possible existence of collision quasi-invariants in the vibron gas. It is proposed that some properties of vibrational spectrum in molecular crystals may lead to a quasi-conservation law of the whole vibron number. A usable expression of the attenuation resulting from such a quasi-invariant is derived and confronted with experimental results. Finally, the ultrasonic relaxation times are discussed and their relation with vibron depopulation times is analysed.