The infra-red absorption of interstitial 17O and 18O was measured, at high resolution, in a multi-reflection geometry. The asymmetrical mode of 18O was seen to be broadened by interaction with the phonon background, while that of 17O was unaffected. The broadening was confirmed by results for an 18O-enriched sample. All of the O and Ge isotope shifts could be fitted, or accurately predicted, by using a 2-parameter semi-empirical model. New low-intensity lines were also observed in the multi-reflection geometry, and were attributed to transitions with a change in the rotational state of 16O between the ground and excited vibrational states. The energies of the 2-dimensional 16O rotator were derived from these measurements, and the Ge isotope effect was noted to be much smaller than that deduced from phonon spectroscopy.

O Isotope Effects and Vibration-Rotation Lines of Interstitial Oxygen in Germanium. B.Pajot, P.Clauws, J.L.Lindström, E.Artacho: Physical Review B, 2000, 62[15], 10165-72