The Parallel Type Absorption Bands of Ammonia

The form of the ammonia molecule is discussed together with the theory that all of the vibrational levels of the molecule should be double due to the two equivalent positions of equilibrium of the nitrogen atom. This theory serves to explain the doubling of the ∥ band at 10.5μ and further predicts that the lines of the 3.0μ band should also appear as doublets although with a much finer separation. An experimental search is made for the doubling of the 3.0μ band using an infrared spectrometer of high resolving power. The effect is found, each fine structure line appearing as a doublet with a mean separation of about 1.6 ${\mathrm{cm}}^{-1\mathrm{}}$. The theoretical intensities of the ∥ type ammonia bands are computed assuming that the hydrogen nucleus possesses a spin of $\frac{1}{2}\left(\frac{h}{2\pi }\right)$. When these values are compared with the observed lines of the 3.0μ and the 10.5μ bands, a very satisfactory agreement is obtained. This agreement furnishes a strong argument for the theory of the doubling of the ammonia bands. It further proves that those states of ammonia existing in nature have vibration—rotation—nuclear spin wave functions which are antisymmetrical for an interchange of two of the hydrogen atoms.