Local vibrational modes of H2 and H2* complexes in crystalline Si

We study the local vibrational properties of H-2 molecules in crystalline Si using a first-principles pseudopotential method within the local-density-functional approximation and the generalized gradient approximation. The dynamical matrix is calculated using a supercell geometry, so that all the normal vibrational modes are identified for the [100], [110] and [111] orientations of the H-2 molecule at or near a tetrahedral site. For the orientations considered here, a new local mode is found at 650-700 cm(-1), which lies above the bulk phonon band, while the vibrational frequencies of the stretch mode are in the range of 3556-3643 cm(-1), close to the experimentally measured value of 3618 cm(-1). However, considering anharmonic effects, the calculated frequencies for the stretch made are expected to be lowered by about 200 cm(-1). We also examine the vibrational frequencies for an H-2* complex, and find the stretch and wagging modes to be in good agreement with experiments.