The Infrared Absorption Spectrum of Carbon Dioxide

Abstract

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 ${\ensuremath{\nu}}_{1}$ appears only in combination bands, but ${\ensuremath{\nu}}_{2}$ at 15\ensuremath{\mu} and ${\ensuremath{\nu}}_{3}$ at 4.3\ensuremath{\mu} absorb intensely.Resolution of the fundamentals ${\ensuremath{\nu}}_{2}$ and ${\ensuremath{\nu}}_{3}$.---The 15\ensuremath{\mu} 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\ensuremath{\mu} 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 15\ensuremath{\mu} 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\ifmmode\times\else\texttimes\fi{}${10}^{\ensuremath{-}40}$ gr ${\mathrm{cm}}^{2}$.Harmonic and combination bands.---No first harmonic bands appear, either for ${\ensuremath{\nu}}_{2}$ or ${\ensuremath{\nu}}_{3}$, but the second harmonic has been observed in each case. The strong pair of doublet bands at 2.7\ensuremath{\mu} are interpreted as combination bands corresponding to ${\ensuremath{\nu}}_{3}+{{\ensuremath{\nu}}_{1}, 2{\ensuremath{\nu}}_{2}}$ and the bands at 2.0\ensuremath{\mu} and 1.6\ensuremath{\mu} are higher members of the same sequence. The differences ${\ensuremath{\nu}}_{3}\ensuremath{-}{{\ensuremath{\nu}}_{1}, 2{\ensuremath{\nu}}_{2}}$ explain the weak absorption maxima observed by Schaefer and Philipps at 9.4\ensuremath{\mu} and 10.4\ensuremath{\mu}. The difference bands ${\ensuremath{\nu}}_{1}\ensuremath{-}{\ensuremath{\nu}}_{2}$ form a part of the 15\ensuremath{\mu} pattern.Correlation with Raman spectra.---The Raman lines corresponding to the transitions $0\ensuremath{\rightarrow}{{\ensuremath{\nu}}_{1}, 2{\ensuremath{\nu}}_{2}}$ and the weaker pair originating in the first excited state ${1}_{1}$ of ${\ensuremath{\nu}}_{2}$ fit precisely into the energy level diagram obtained from infrared measurements.