Diffusion and ortho-para conversion in solid hydrogen

The ortho-para conversion in solid hydrogen is investigated at 12, 4.2, and 1.57 K in long-term experiments up to 1200 h. The results are discussed in the light of the influence of molecular diffusion on the conversion rate. At 4.2 and 1.57 K, the conversion rate shows that the diffusion coefficient is $D\approx {10}^{-20\mathrm{}}$ ${\mathrm{cm}}^2$ ${\sec }^{-1\mathrm{}}$. This value is much larger than those predicted from an extrapolation of experimental results at higher temperatures and from recent theoretical predictions. At 1.57 K, an increase of the conversion rate with decreasing $o-{\mathrm{H}}_2$ concentration is observed. Calculations show this increase is due to clustering of $o-{\mathrm{H}}_2$ molecules caused by electric quadrupole-quadrupole interactions between them. The estimated diffusion rate is consistent with that found previously from the rate of clustering.