Interstitial H2 in Si: are all problems solved?

Interstitial H2 in Si has given rise to a number of perplexing puzzles since the discovery of its vibrational spectrum. The absence of an ortho-para splitting for the H2 line and an apparent low-symmetry found in stress experiments misled several researchers, including two of the present authors, into thinking that interstitial H2 must have a barrier that prevents rotation. Our discovery of a new vibrational line for interstitial HD in Si and its interpretation establish that interstitial H2 in Si is a nearly free rotator. The insights provided by these results have led to simple, in retrospect, explanations of the microscopic properties of interstitial H2 and an O-H2 complex in Si. Nonetheless, interesting new puzzles have arisen. The results of a recent Raman study have led to the suggestion that ortho- and para-H2 in Si have different diffusivities. In contrast to this suggestion, new results for the formation of complexes of interstitial oxygen with para-D2 and ortho-D2 in Si suggest that the para and ortho species have similar diffusivities at room temperature