O isotope effects and vibration-rotation lines of interstitial oxygen in germanium

The infrared absorption of interstitial ¹⁷O and ¹⁸O has been measured at high resolution in natural germanium in a multireflection geometry. The asymmetric mode of ¹⁸O is observed to be broadened by interaction with the phonon background while that of ¹⁷O is unaffected. The broadening is confirmed by results on an ¹⁸O-enriched sample. All the O and Ge isotope shifts can be fitted or predicted rather accurately by a two-parameter semiempirical model using values determined by Pajot and Clauws in 1988. In the multireflection geometry, new lines of low intensity are also observed and they are ascribed to transitions with change of the rotational state of ¹⁶O between the ground and excited vibrational states. Energies of the two-dimensional ¹⁶O rotator are derived from these measurements, with a Ge isotope effect much smaller than the one deduced from phonon spectroscopy.