The far-infrared absorption spectrum of low temperature hydrogen gas
Creators: name: Wishnow, Edward H.
Publisher: name: University of British Columbia
Year: 2008

description: The far-infrared absorption spectrum of normal hydrogen gas has been measured from 20-320 cm' (A=500-31 pm), over the temperature range 21-38 K, and the pressure range0.6-3 atmospheres. The spectra cover the very weak and broad collision-induced translational absorption band of 112 which at these low temperatures is observed well isolated from the 112 rotational lines. Translational absorption occurs when two molecules collide and absorb a photon via a transient induced dipole moment. The molecules emerge from the collision with altered translational energies, and the rotational, vibrational, and electronic energy states remain unaffected. The present spectra are the lowest temperature, lowest pressure, and highest resolution studies of the 112 translational spectrum. In order to observe the weak translational absorption band, a long path length multi-reflection absorption cell (White cell'), cooled by the continuous flow of helium vapour, has been designed and constructed. The cell has an f/10 optical beam that allows long wavelength radiation to be transmitted, with low diffraction losses, over an optical path of up to 60 m. The cell is coupled to a Fourier transform interferometer and 112 spectra are obtained at a spectral resolution of 0.24 cm-1, 10 times higher than previous experiments. Low temperature absorption spectra are due to not only transitions between molecular translational energy states, but also rotational transitions between the bound states of the van der Waals complex formed by two hydrogen molecules. The integrated absorption of the measured 112 translational spectrum is consistent with the binary absorption coefficient calculated using the Poll and Van Kranendonk theory of collision-induced absorption. The calculation is based on the quantum mechanical pair distribution function derived from the Lennard-Jones intermolecular potential, and it includes contributions from 112 dimer bound states. Although dimer transitions contribute to the translational continuum, no sharp spectral features are observed that can be attributed to dimer transitions. These low temperature, low pressure spectra are the first experimental searches for dimer structure across the translational band. The 112 spectra show the sharp R(0) line of HD superposed on the translational band at 89.2 cm', where the HD is present in the gas in natural abundance. The electric dipole moment of HD has been measured from the intensity of this line and it equals(0.81 + 0.05) x 10' debye. The low temperature 112 spectra provide an experimental test of the theory of collision-induced absorption and the model of the 112 intermolecular potential. Furthermore, since the present experimental conditions are close to those found in the atmospheres of the planets Uranus and Neptune, these spectra also have relevance to astronomical observations of 112. This work demonstrates, in particular, the feasibility of using measurements of the HD R(0) line and the underlying 112 translational band to obtain planetary D/H ratios. In addition to hydrogen, spectra of H2-He mixtures, D2, N2, 02; and mixtures of N2,CH4, and CO with Ar, have been obtained at 78 and 88 K over the spectral region 20-180cm'. The N2 and N2-Ar spectra exhibit structure superposed on the collision-induced translational-rotational band; this structure has not been previously observed and it is probably due to dimer transitions.