The simultaneous transformation of both components of the complex isotropic vibrational susceptibility, which occurs with an increase in the density of a gas consisting of linear molecules, is traced for the
first time in terms of the quantum model of strong collisions. The vibrational susceptibility of the CO2 molecule in the region of the fundamental ν3 band is discussed in detail as a case in point. It is found that within
the region of anomalous dispersion adjacent to frequency ν3, there is a spectral interval that is characterized
by positive dispersion and exists in a fairly wide pressure range. This interval narrows upon an increase in
pressure and disappears at a density of around 100 Amagat, completely blurring the branch structure of the
absorption band. It is shown that allowing for the spectral exchange between the lines of the rotational structure of the band is equally important for quantitatively interpreting the spectrum of the real part of susceptibility and reproducing a correct picture of the transformation of the absorption spectrum.