Ultrasonic Propagation in Solid Methane

Abstract

We present here a preliminary account of recent measurements of the velocity and attenuation of sound in solid methane, CH₄. The work was motivated in part by studies of the proton magnetic resonance in CH₄ by Thomas, Alpert, and Torrey.¹ Using pulse methods, they measured the spin-lattice relaxation time T ₁, and found that it exhibits an anomalous peak at 20.4°K, the temperature of the (upper) phase transition. Their results over the temperature region above and below the peak have been analyzed by Tomita,² who derived a relationship between T₁ and the average time τ _c spent by a molecule in one of its equivalent orientations in the lattice before it flips to another. Near T _λ , τ _c ≈ 10^-7 sec, corresponding to a frequency spectrum that extends over a range of several megacycles. We were led to a study of the sound velocity and attenuation at frequencies of this order of magnitude to look for evidence, direct or indirect, of coupling between the rotational motions of the molecules and the vibrations of the lattice. In this paper, we show that anomalies in both the velocity and absorption do in fact occur near the λ-point, and that the relaxation time characterizing the absorption is of the same order of magnitude as the molecular flipping time τ _c . We have not yet been successful, however, in establishing a relationship between our results and those of the NMR investigations.

Supported in part by the Advanced Research Projects Agency under Contract SD-90, and in part by the Research Laboratory of Electronics, M.I.T., supported jointly by U.S. Army Signal Corps, Office of Naval Research, and Air Force Office of Scientific Research under Contract DA36-039-sc-78108.

Supported by the U.S. Air Force Office of Scientific Research.