The Infrared Absorption Spectrum of Carbon Dioxide

The complete infrared spectrum of C${\mathrm{O}}_2$ 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 ${\nu }_1$ appears only in combination bands, but ${\nu }_2$ at 15μ and ${\nu }_3$ at 4.3μ absorb intensely.

Resolution of the fundamentals ${\nu }_2$ and ${\nu }_3$.—The 15μ 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 4.3μ band has also been resolved, best results following a material reduction in the C${\mathrm{O}}_2$ content of the atmospheric path. The line spacing is the same as in the 15μ 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×${10}^{-40\mathrm{}}$ gr ${\mathrm{cm}}^2$.

Harmonic and combination bands.—No first harmonic bands appear, either for ${\nu }_2$ or ${\nu }_3$, but the second harmonic has been observed in each case. The strong pair of doublet bands at 2.7μ are interpreted as combination bands corresponding to ${\nu }_3+\{{\nu }_1, 2{\nu }_2\}$ and the bands at 2.0μ and 1.6μ are higher members of the same sequence. The differences ${\nu }_3-\{{\nu }_1, 2{\nu }_2\}$ explain the weak absorption maxima observed by Schaefer and Philipps at 9.4μ and 10.4μ. The difference bands ${\nu }_1-{\nu }_2$ form a part of the 15μ pattern.

Correlation with Raman spectra.—The Raman lines corresponding to the transitions $0\to \{{\nu }_1, 2{\nu }_2\}$ and the weaker pair originating in the first excited state $1_1$ of ${\nu }_2$ fit precisely into the energy level diagram obtained from infrared measurements.