The Pure Rotation Spectrum of Ammonia

Abstract

New experimental means for the study of absorption spectra in the extreme infra-red are described. These have been used in extending our knowledge of the pure rotation spectra to the case of a polyatomic molecule. The absorption spectrum of ammonia has been investigated in the region between 55μ and 130μ, and a very simple structure was found. Six lines were observed which belong to a pure rotation spectrum and are apparently due to changes in the energy of rotation of the ammonia molecule about an axis normal to the line of symmetry, that is, to transitions in which the quantum number $j$ increases by unity. Other lines due to transitions with a change also of $\tau$, the quantum number connected with rotations about the axis of symmetry, are absent. These facts are briefly discussed in connection with the predictions of the wave mechanics with which they are shown to be in accord. The moment of inertia of the ammonia molecule about an axis normal to the line of symmetry is estimated to be 2.77×${10}^{-40\mathrm{}}$ gm ${\mathrm{cm}}^2$.