The Molecular Spectrum of Ammonia I. Two Types of Infra-Red Vibration Bands

Wave-lengths and interpretation of the NH₃ absorption bands at 3.0 and 1.9{μ}---The NH₃ absorption bands at 3.0{μ} and at 1.9{μ} have been sufficiently resolved by means of gratings to reveal their fine structure, which proves to be of a different character in the two cases. The 3{μ} band is associated with a vibration of the electric moment parallel to the symmetry axis of the molecule, yielding one zero branch and a simple rotation series. At 1.9{μ} there is observed a series of almost equally spaced lines showing a remarkable uniformity in intensity so that the envelope is a single broad maximum instead of the usual doublet. The band center cannot be selected by inspection. The vibration which gives rise to this band is normal to the symmetry axis. From the 3{μ} band the moment of inertia of the molecule (I=2.83×10^-40) about a line normal to the symmetry axis may be determined, but this does not yield the distances between atomic nuclei. The 1.9{μ} band probably corresponds to one of the four fundamental frequencies. Because of its character it cannot be a harmonic nor a combination of vibrations along the axis as had previously been supposed. The 10{μ} and the 6{μ} bands probably correspond to two other fundamentals, but the selection of the fourth fundamental, and the characterization of combination bands must await further analysis to determine the nature of the vibrations involved.