Roton relaxation in parahydrogen crystals measured by time-resolved stimulated Raman gain

Picosecond time-resolved dephasing measurements of the Raman-active J=2 rotons in para-${\mathrm{H}}_2$ single crystals have been performed with the stimulated-Raman-gain technique. The three distinct J=2 rotons (‖$M_J$‖=0, 1, or 2) could be selected by a proper choice of the polarizations of the four laser beams with respect to the crystal axes. Excellent exponential fits were obtained for the decay curves, indicating little or no inhomogeneous broadening. The resulting $T_2$ times exhibit no temperature dependence between 5 and 10 K and are in good agreement with recent high-resolution spectral measurements. However, the present time-domain results are more reliable and accurate. In the explored range of ortho-${\mathrm{H}}_2$ impurity concentration of up to 5%, a linear increase is found for the relaxational linewidths (∼1/$T_2$) with nearly equal slopes for the different ‖$M_J$‖ rotons. These results extrapolated to pure para-${\mathrm{H}}_2$ yield $T_2$ times in the neighborhood of 100 ps which agree quite well with those of a recent microscopic calculation. However, in the experiments unpredicted and significant differences (up to 30%) are evident between $T_2$ times for different ‖$M_J$‖ values. The rotational relaxation was also measured in the liquid phase of para-${\mathrm{H}}_2$ and a value $T_2$=7.5 ps was obtained. It too showed a small decrease with increasing ortho-${\mathrm{H}}_2$ concentration.