Ortho-para conversion of isolated interstitial ${\mathrm{H}}_2$ in single-crystalline Si is studied by Raman scattering. This process is suggested to be caused by the interaction of ${\mathrm{H}}_2$ with the nuclear magnetic moment of ${}^{29}\mathrm{Si}$. At 77 K the ortho-to-para conversion rate is approximately $0.015{\mathrm{h}}^{-1}$ for all Si samples employed in the experiments. At 300 K, the reverse para-to-ortho transition is observed. It occurs with a rate of roughly $0.18{\mathrm{h}}^{-1}$ and results in a thermodynamically nonequilibrium ortho-para ratio.

• Received 24 October 2006

DOI:https://doi.org/10.1103/PhysRevLett.98.055504