Dispersed oxygen in Ge leads to the formation of Ge2O ''quasimolecules'' whose asymmetric stretching mode (ν3), exhibits a remarkable, temperature-dependent fine structure when examined under the ultrahigh resolution of a Fourier-transform spectrometer. The large number of sharp lines observed arises from the coupling of ν3 with the low-energy symmetric bending ν2 mode; the thermal population of the ν2 levels, together with the shifts associated with the isotopes of Ge in their natural abundances, underlie its fine structure and temperature dependence. By incorporating oxygen in monoisotopic Ge70, Ge73, Ge74, or Ge76, the excitation spectrum of the ν2+ν3 coupling is selectively simplified. From the temperature dependence of the intensities of the lines in the fine structure, the energies of the ν2 levels are deduced. The O18 counterpart of the ν2+ν3 coupling excitation has been observed and a consistent interpretation of the ν2 levels as well as the shifts of the ν2+ν3 coupling excitations associated with any isotopic combination of oxygen and Ge is obtained by using an interaction mass m'=23.1 amu and a Ge-O-Ge bond angle 2α=111°.
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