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

Abstract

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\ensuremath{\mu} has been applied in the spectral range from 60 to 125\ensuremath{\mu}. The resolving power was several times that hitherto obtained in this region. The pure rotation absorption spectra of water vapor and of the gases N${\mathrm{H}}_{3}$ and P${\mathrm{H}}_{3}$ have been investigated. In the case of N${\mathrm{H}}_{3}$ 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 ${\mathrm{cm}}^{\ensuremath{-}1}$. Despite the similarity of structure of the molecules P${\mathrm{H}}_{3}$ and N${\mathrm{H}}_{3}$ the pure rotation lines of P${\mathrm{H}}_{3}$ exhibit no trace of a doubling. The frequencies of the P${\mathrm{H}}_{3}$ lines and the mid-frequencies of the N${\mathrm{H}}_{3}$ doublets have been shown to agree very accurately with formulas of the type ${{\ensuremath{\nu}}_{J}}^{J\ensuremath{-}1}=BJ\ensuremath{-}D{J}^{3}$. Determinations of the moments of inertia $A$ of N${\mathrm{H}}_{3}$ and P${\mathrm{H}}_{3}$ have been made and in the case of N${\mathrm{H}}_{3}$ 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.