The Far Infra-Red Absorption Spectrum and the Rotational Structure of the Heavy Water Vapor Molecule

Abstract

An investigation has been made of the spectrum of heavy water vapor (${\mathrm{D}}_2$O) in the region from 23μ to 135μ. The instrument used was a self-recording spectrograph of large aperture, using echelette gratings, vacuum thermopile, and a system of filters, shutters, and reststrahlen plates to remove higher order spectral impurity. The radiation path in the spectrograph could be evacuated. From this research the experimental positions and relative intensities of 210 pure rotation absorption frequencies were obtained. Absorption maxima were located with an accuracy of about 0.05 ${\mathrm{cm}}^{-1\mathrm{}}$. Lines 0.5 ${\mathrm{cm}}^{-1\mathrm{}}$ apart were partially resolved, higher resolution and dispersion being of little advantage since the true width of these absorption lines was of this same order of magnitude. The energy levels of a zeroth-order approximation to the ${\mathrm{D}}_2$O asymmetric rotator molecule were computed through quantum number $j=11\mathrm{}$, and corrected for zero point vibration and centrifugal stretching in the ground state. A comparison of the positions and intensities of the experimental data with those of the transitions between these "key" levels showed a rather good agreement. These levels were therefore corrected to fit the data, and checked for consistancy by means of series regularities and combination relations. In all, 111 distinct energy levels based on the experimental data were computed. A graph of the experimental data contrasted with a similar graph of the transitions based on these corrected levels gives a clear picture of the success of the analysis.