The rotational structure of two absorption bands of C${\mathrm{O}}_2$ at 9.4μ and 10.4μ has been completely resolved. Both bands have positive and negative branches, but no zero branches. They correspond to transitions up to the state ${\nu }_3$ from the double state (${\nu }_1, 2{\nu }_2$), and yield the value 102.8 ${\mathrm{cm}}^{-1\mathrm{}}$ for the separation of the two levels. From the intensity distribution of the lines it is clear that $J$ must have only even values in the normal molecule. The line separations and convergences provide a very precise value for the moment of inertia, namely 70.6×${10}^{-40\mathrm{}}$ gram ${\mathrm{cm}}^2$. The numerical agreement with predictions from formulae based upon previous measurements with other bands is very satisfactory.

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