The Molecular Spectrum of Ammonia II. The Double Band at 10μ

Abstract

Structure of the N${\mathrm{H}}_{3}$ absorption band at 10\ensuremath{\mu}.---The N${\mathrm{H}}_{3}$ absorption band extending from 8\ensuremath{\mu} to 14\ensuremath{\mu} has been sufficiently resolved by means of a grating to reveal the details of its structure. Two very narrow zero branches of nearly equal intensity appear at 10.3\ensuremath{\mu} and 10.7\ensuremath{\mu}, with each of which there is associated a positive and a negative branch consisting of ten or twelve lines spaced at nearly equal frequency intervals. The mean spacing for the higher frequency series is 18.7 ${\mathrm{cm}}^{\ensuremath{-}1}$, and for the lower frequency series 20.2 ${\mathrm{cm}}^{\ensuremath{-}1}$. Both of these bands must be associated with a single fundamental vibration parallel to the symmetry axis.Configuration of the molecule as revealed by intensities in the 1.9\ensuremath{\mu} band.---The band structure corresponding to a vibration normal to the symmetry axis is described for symmetrical rotators of the oblate and prolate forms. Line intensities are computed for each from known transition probabilities, and only the oblate form yields intensities in accord with those observed at 1.9\ensuremath{\mu}. The distance of the N atom from the plane of the three H atoms apparently lies between 0.1\ifmmode\times\else\texttimes\fi{}${10}^{\ensuremath{-}8}$ and 0.3\ifmmode\times\else\texttimes\fi{}${10}^{\ensuremath{-}8}$ cm, which is approximately one fifth of the distance between H atoms.Interpretation of the double band.---The double character of the 10\ensuremath{\mu} band is shown to be a consequence of the close proximity of the two equilibrium positions for the N atom, one on either side of the plane formed by the H atoms.