Ortho-para transition of interstitialH2andD2in Si

Abstract

The ortho-para transition of interstitial ${\text{H}}_{2}$ in Si has been observed in a recent Raman study [M. Hiller et al., Phys. Rev. Lett. 98, 055504 (2007); M. Hiller et al., Phys. Rev. Lett. 99, 209901 (2007)]. In order to address issues that are difficult to study by Raman spectroscopy, we have performed IR-absorption experiments to further investigate this ortho-para transition. We find that when a Si sample containing ${\text{H}}_{2}$ that has been equilibrated at room temperature is subsequently held at 77 K, the $3618.4\text{ }{\text{cm}}^{\ensuremath{-}1}$ IR line assigned to ortho-${\text{H}}_{2}$ is reduced in intensity with a single exponential time constant of 229 h as the system relaxes to the para-${\text{H}}_{2}$ state. The previous Raman data had been analyzed by a model in which the ortho-para transition is caused by an interaction with the nuclear magnetic moment of $^{29}\text{S}\text{i}$ (4.7% abundant) that is present in natural Si. However, in order for every ${\text{H}}_{2}$ molecule to interact with a $^{29}\text{S}\text{i}$, at least six shells of Si neighbors must be considered. Because of the strong dependence of the transition rate on the $^{29}\text{S}{\text{i-H}}_{2}$ distance, the kinetics of such a transition should be highly nonexponential. Thus, our IR data are not consistent with this mechanism. We have also investigated the para-ortho transition of interstitial ${\text{D}}_{2}$ and have found decay rates similar to those for the ortho-para transition of ${\text{H}}_{2}$. We have considered two possible mechanisms for this transition: ${\text{H}}_{2}$ or ${\text{D}}_{2}$ tunneling, which would allow the molecule to sample tetrahedral sites adjacent to $^{29}\text{S}\text{i}$, and spin-rotation coupling that can act while the molecule is dynamically off-center. Consideration of a possible tunneling rate by theory and the similarity of the ${\text{H}}_{2}$ and ${\text{D}}_{2}$ results seem to rule out the tunneling mechanism but not the spin-rotation mechanism. These results clearly show that the mechanism of the ortho-para transition for ${\text{H}}_{2}$ and ${\text{D}}_{2}$ in Si remains an experimental and a theoretical challenge.