Sound propagation and vibrational relaxation in molecular crystals

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