Comparison between the density effects on the levels of the Raman spectra of the Fermi resonance doublet of the 12C16O2 and 13C16O2 molecules

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Abstract
Isotropic Raman studies of density effects on the lower Fermi doublet ν1, 2ν2 of the 12C16O2 and 13C16O2 compressed molecules have been made in the gaseous and liquid phases along the saturation line up to the triple point temperature. After a brief bibliography review of the unperturbed frequency ordering for the 12C16O2 case, the problem of the determination of unperturbed levels from our experimental data is treated for both isotopic species. Comparison between 12C and the 13C frequency shifts due to the density effect leads to the conclusion that the order of the ν1, 2ν2 levels is reversed in the liquid and gaseous phase of 12C16O2 with an exact Fermi resonance ν1=2ν2 in the high‐density fluid phase. For the first time, bandwidths of the two components of the Fermi dyad are measured in the liquid.