The ortho-para conversion of interstitial ${\mathrm{H}}_2$ in single-crystalline natural Si (${}^{\text{nat}}\mathrm{Si}$) and ${}^{29}\mathrm{Si}$ (enrichment 96.2%) is studied by Raman scattering. The conversion process was found to be practically independent of the isotope composition of Si. The characteristic ortho-to-para conversion time at 77 K was found to be $220\pm 35$ and $200\pm 35$ h for ${}^{29}\mathrm{Si}$ and ${}^{\text{nat}}\mathrm{Si}$, respectively, whereas at room temperature the back conversion occurs with a characteristic time of $8.8\pm 1.5$ and $10.5\pm 1.5$ h for ${}^{29}\mathrm{Si}$ and ${}^{\text{nat}}\mathrm{Si}$, respectively. These values agree very well with the transition rates found in previous IR absorption studies by Peng et al. for ${}^{\text{nat}}\mathrm{Si}$ [Phys. Rev. B 80, 125207 (2009)]. Our findings imply that an interaction of ${\mathrm{H}}_2$ with the nuclear spin of nearby ${}^{29}\mathrm{Si}$ has marginal, if any, effects on the ortho-para conversion rate of molecular hydrogen in silicon.

• Received 19 March 2021
• Revised 14 April 2021
• Accepted 5 May 2021

DOI:https://doi.org/10.1103/PhysRevB.103.205204