Hydrogen molecules in silicon located at interstitial sites and trapped in voids

The vibrational modes of H-2 molecules in Si are found using a first-principles method and compared with recent experimental investigations. The isolated molecule is found to lie at a T-d interstitial site, oriented along [011] and is infrared active. The rotational barrier is at least 0.17 eV. The molecular frequency is a sensitive function of cage size and increases to lie close to the gas value for cages about 50% larger than the T-d site. It is suggested that Raman-active modes around 4158 cm(-1) are due to molecules within voids.