The strange behavior of interstitial H2 molecules Si and GaAs

Interstitial H-2 molecules have been observed in GaAs and in Si and behave very differently in the two hosts. In GaAs, H-2 has rotational symmetry, a stretch frequency close to the free-molecule value, and the Raman spectrum exhibits the expected ortho/para splitting. In Si, the symmetry is C-1, the stretch frequency is about 550 cm(-1) lower than that of free H-2, and no ortho/para splitting is seen. In this paper, we use ab-initio molecular-dynamics simulations (SIESTA) to calculate the vibrational frequencies of H-2 using linear response theory as well as the velocity-velocity autocorrelation function. We also study the interactions between H-2 and O-i as well as paramagnetic H in Si. Based on these calculations and arguments about the formation process of H-2, we argue that rapid H(2)reversible arrowH(T)(0) interactions are a possible candidate to explain the absence of ortho-H-2 in Si. (C) 2001 Elsevier Science B.V. All rights reserved.