The Infrared Absorption Spectrum of Carbon Dioxide

The complete infrared spectrum of CO₂ may consistently be explained in terms of a linear symmetrical model, making use of the selection rules developed by Dennison and the resonance interaction introduced by Fermi. The inactive fundamental ν₁ appears only in combination bands, but ν₂ at 15{μ} and ν₃ at 4.3{μ} absorb intensely.Resolution of the fundamentals ν₂ and ν₃.---The 15{μ} band has been resolved into several constituent bands corresponding to absorption by the normal molecule, and by molecules in the first and second excited states. Each band consists of a narrow and intense zero branch, with equally spaced rotation lines on either side. The 4.3{μ} band has also been resolved, best results following a material reduction in the CO₂ content of the atmospheric path. The line spacing is the same as in the 15{μ} band, and there is no zero branch. In each case alternate rotation lines are absent, the J values for the normal state all being even. The computed moment of inertia is 70.8×{}10^-40 gr cm².Harmonic and combination bands.---No first harmonic bands appear, either for ν₂ or ν₃, but the second harmonic has been observed in each case. The strong pair of doublet bands at 2.7{μ} are interpreted as combination bands corresponding to ν₃+ν₁, 2ν₂ and the bands at 2.0{μ} and 1.6{μ} are higher members of the same sequence. The differences ν₃-ν₁, 2ν₂ explain the weak absorption maxima observed by Schaefer and Philipps at 9.4{μ} and 10.4{μ}. The difference bands ν₁-ν₂ form a part of the 15{μ} pattern.Correlation with Raman spectra.---The Raman lines corresponding to the transitions 0→ν₁, 2ν₂ and the weaker pair originating in the first excited state 1₁ of ν₂ fit precisely into the energy level diagram obtained from infrared measurements.