Brillouin spectroscopic studies of xenon and methane hydrates

Abstract

The technique of Brillouin Spectroscopy has been used to study the acoustic properties of clathrate hydrates. In particular, the longitudinal sound velocities in the hydrates of methane and xenon have been determined from artificially-grown polycrystalline samples. -- A major part of this study involved an investigation of procedures for producing appropriate samples. This was met with limited success. Except for one xenon hydrate sample, the samples grown were somewhat opaque and milky. Nevertheless, light scattering studies were successfully carried out. -- Since an exact measurement of the percentage cage occupancy was not feasible, the data are presented at occupancies of 75%, 90% and 100%. The dependency of the velocities on the occupancies arose through the Brillouin equation wherein the calculated hydrate indices of refraction were dependent on the occupancy percentage through the hydrate densities. Further, each velocity is expressed as a ratio relative to the longitudinal velocity in ice. Two sets of data for xenon hydrate are presented. The results are as follows: -- HYDRATE - CAGE OCCUPANCY - HYDRATE/ICE VELOCITY RATIO -- METHANE - 75 - .894 -- 90 - .887 -- 100 - .880 -- XENON - 75 - .775 - .761 -- 90 - .764 - .750 -- 100 - .753 - .739 -- These data represent the first experimental velocity measurements for methane and xenon hydrates. The velocity ratios obtained through an extension of the only theoretical treatment performed to date (Whalley, 1980) are in approximate agreement.