Deciphering the Vibrational Spectrum of Interstitial H2 in Si

Abstract

ABSTRACTH2 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 H2 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 H2 vibrational line and an apparent low symmetry found in stress experiments misled several researchers into thinking that interstitial H2 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 H2 or D2, establish that H2 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 H2 and D2 vibrational lines, and the properties of an O-H2 complex are also explained naturally. Recent Raman studies confirm that interstitial H2 in Si is a free rotator but raise interesting new questions about the diffusivities of the ortho and para species.