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

The vibrational modes of ${\mathrm{H}}_2$ 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 ${\mathrm{cm}}^{-1}$ are due to molecules within voids.