Ortho-para transition of interstitial H$_{2}$ in Si

Interstitial H$_{2}$ in Si is a nearly-free rotator and has ortho and para species with the nuclear spins of the two protons aligned either parallel or antiparallel [1]. If one waits a sufficiently long time at low temperature, H$_{2}$ will relax to its lower energy para state. The ortho-para (o-p) transition for H$_{2}$ in Si has been observed in recent Raman studies [2]. We have performed IR absorption experiments to investigate issues that have proved difficult to study by Raman. When a Si sample containing H$_{2}$ is stored for a month or more at 77K, the 3618.4 cm$^{-1}$ IR line assigned to o-H$_{2}$ [1] is reduced in intensity because, when the o-p transition occurs, p-H$_{2}$ is not seen by IR. When this sample was annealed at room temperature, the ortho population characteristic of room temperature was recovered with a time constant of $\approx $6 hrs. Our IR studies of the kinetics of the o-p transition complement recent Raman results and suggest that the cause of the o-p transition is not yet understood. \newline [1] M. Stavola \textit{et al.}, Physica B \textbf{340-342}, 58 (2003). \newline [2] M. Hiller \textit{et al.}, Phys. Rev. Lett. \textbf{98}, 055504 (2007); \textbf{99}, 209901 (2007).