However, there is an additional mode seen at 1105 cm-1 in the presence of the dimer modes , and it was speculated that this may be two Oi atoms seperated by a single Si-Si bond along 110 . An alternative explanation is that this is due to the symmetric dimer. Although the calculated symmetric dimer does not have a mode exactly at this value, there is some evidence in favour of this model for the 1105 cm-1 mode. The defect appears to have an experimental binding energy of 0.2 eV as compared to 0.3 eV for the dimer , so an energy difference of 0.1 eV between the two structures is quite close to our calculated value of 0.26 eV. The mode is only observed at 10K, and disappears by 300K. This would be consistent with symmetric dimers getting `frozen in' at lower temperatures with a finite reorientation barrier. At 300K they are able to overcome this and no symmetric dimers are observed. In addition the 1105 cm-1 mode drops by 50/52 cm-1 when 16O is switched to 18O, very close to our calculated value of 52.6 cm-1. To fully check this model it would be necessary to calculate the energy barrier between the symmetric and assymmetric structures, as well as locate further experimental modes to improve the assignment.