Interstitial H2 became bound to interstitial O in Si, forming an O-H2 complex. Infrared absorption experiments performed for the O-HD complex in Si reveal HD vibrational lines that appeared at above 10K. These lines and their properties led to the conclusion that the H2 molecule in the O-H2 complex was a nearly free rotor. Two near-lying lines seen for the O mode of the O-H2 complex were attributed to the ortho and para O-H2 complexes. The ortho and para states of the O-H2 complex also give rise to H2 vibrational
lines with distinct properties. These results could be understood in terms of a hindered-rotor model for the H molecule.

Ortho and Para O-H2 Complexes in Silicon. E.E.Chen, M.Stavola, W.B.Fowler: Physical Review B, 2002, 65[24], 245208 (9pp)