Previous work on oxygen in silicon has shown that oxygen dissolves interstitially in silicon forming a complex which may be approximately described as Si
2
O. Absorption bands of Si : O occur at 517, 1136 and 1203 cm
-1
and these have been assigned by earlier authors to the
v
2
(symmetric bending),
v
3
(antisymmetric stretch) and
v
1
(symmetric stretch) normal modes of vibration of Si
2
O. The present investigation confirms the
v
3
origin of the 1136 cm
-1
band (the well-known 9
μ
m band) but we disagree with the earlier assignments of the 517 and 1136 cm
-1
bands. The results reported here are relevant to organic siloxanes. We have extended the investigation of Si : O into the far infrared and we find sharp absorption lines at 29.3, 37.8, 43.3 and 49.0 cm
-1
which we have assigned to the
v
2
mode of Si
2
O. The isotope shift due to
l8
O has been observed in the far infrared spectrum. Effects of uniaxial stress on the 29.3 cm
-1
line have been investigated and are found to be consistent with the assignment to the
v
2
mode. The main features of the far infrared spectrum are accounted for with a simple anharmonic potential which ignores coupling of the Si
2
O to the crystal lattice. We have investigated effects of uniaxial stress on the 517, 1136 and 1203 cm
-1
bands of Si
2
O. Our stress results for the 1136 cm
-1
band are consistent with the earlier
v
3
assignment. Using our normal mode description, we conclude that the 1203 cm
-1
band is a combination band involving
v
3
and
v
2
excitations. We have not been able to give a clear cut assignment to the 517 cm
-1
band, but we suggest that
v
1
type excitation may be involved. The appendix describes the stress splitting of the 836 cm
-1
band of the silicon A centre in electron irradiated Si : O and our results confirm an earlier model for this centre. In all cases investigated here, the stress splittings arise from raising the orientational degeneracy of the oxygen complex.
Far‐Infrared Spectrum and Hindering Potential of Deuterium Peroxide