Interstitial H2 in Si:: Solution of the puzzle

Interstitial H-2 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 H-2 line and an apparent low-symmetry found in stress experiments misled several researchers, including the present authors, into thinking that interstitial H-2 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 H-2 in Si is a nearly free rotator. The insights provided by these results lead to simple, in retrospect, explanations of the microscopic properties of interstitial H-2 and an O-H-2 complex in Si.