Relaxation Times ofk=0Rotons in Pure Parahydrogen Crystals and Roton Scattering by Orthohydrogen Impurities

A high-resolution Raman investigation is presented of the linewidth of the $J=2\mathrm{}$ rotons in solid $p-{\mathrm{H}}_2$ measured by means of a tandem interferometer-monochromator. The magnitude of the linewidths (≅ 0.08 ${\mathrm{cm}}^{-1\mathrm{}}$) at extremely low $o-{\mathrm{H}}_2$ concentration is in good agreement with a recent microscopic calculation, but a variation of the experimental widths by a factor of 2 was observed for different $\left|M_J\right|$. The dependence of the roton linewidth on the concentration of $o-{\mathrm{H}}_2$ impurities is found to be linear, indicating that the $o-{\mathrm{H}}_2$ act as noninteracting random scattering centers.