Deciphering the vibrational spectrum of interstitial H2 in si

H-2 is a fascinating molecule whose properties revealed the influence of nuclear spin on the molecular wave function in the 1920s. As an interstitial defect in Si, the H-2 molecule 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 vibrational line and an apparent low symmetry found in stress experiments misled several researchers into thinking that interstitial H-2 in Si must have a barrier to rotation. Our discovery of a new vibrational line for HD in Si and its interpretation, along with the recognition that certain transitions are possible for HD, but not for H-2 or D-2, establish that H-2 in Si is a nearly free rotator after all. Additional puzzles such as the anomalous intensity of the HD line, the absence of an isotope dependence for the uniaxial stress splitting of the H-2 and D-2 vibrational lines, and the properties of an O-H-2 complex are also explained naturally. Recent Raman studies confirm that interstitial H-2 in Si is a free rotator but raise interesting new questions about the diffusivities of the ortho and para species.