The Far Infrared Absorption Spectra of Ammonia and Phosphine Gases under High Resolving Power

A spectrometer of large aperture employing a reflection grating of the echelette type ruled to concentrate radiation in the region of the 1st order of 90 has been applied in the spectral range from 60 to 125. The resolving power was several times that hitherto obtained in this region. The pure rotation absorption spectra of water vapor and of the gases NH₃ and PH₃ have been investigated. In the case of NH₃ the absorption lines shown as single in the observations of Badger and Cartwright have been found under the higher resolution of the present work to be doublets with a doublet separation of 1. 33 cm^-1. Despite the similarity of structure of the molecules PH₃ and NH₃ the pure rotation lines of PH₃ exhibit no trace of a doubling. The frequencies of the PH₃ lines and the mid-frequencies of the NH₃ doublets have been shown to agree very accurately with formulas of the type {₉}^J-1=BJ-DJ^3. Determinations of the moments of inertia A of NH₃ and PH₃ have been made and in the case of NH₃ the measurement of the doublet separation has led to a slight revision of the values for the moment of inertia C and for the molecular dimensions obtained by Dennison and Uhlenbeck.