In order to verify a model for the local-mode coupled low-energy excitation of O, high-resolution far infra-red absorption measurements were made of O lines at 29.3, 37.7, 43.3 and 48.6/cm. The attribution of these lines to certain optical transitions, previously based upon the model, was here supported by the measured temperature dependences of the line intensities. The model, which explained the line energies, was also successful in describing the temperature dependence of the line-intensity ratio. This ratio was shown to be a manifestation of the quartic anharmonicity (off-center nature) of the potential for the low-energy excitation.

Far-infrared absorption by interstitial oxygen impurities in silicon crystals H.Yamada-Kaneta: Physical Review B, 1998, 58[11], 7002-6