Infrared Spectra of HCl and DCl in Solid Rare Gases. I. Monomers

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Abstract
A low‐temperature infrared cell capable of maintaining temperatures between 7° and 50°K is used for the study of the vibration—rotation spectra of HCl and DCl in solid rare gases. Spectra at 0.2% to 2% concentrations in solid krypton and xenon are obtained. Spectra in argon have already been reported by other workers. Fine structure in the region of the (1–0) vibrational transitions is assigned to monomeric and polymeric species of HCl and DCl. The polymer lines can be assigned to dimers and trimers. A detailed description of these interesting features will appear in Paper II of this series. The monomer lines are assigned to vibration—rotation transitions of HCl and DCl. In addition to a J‐independent band‐origin shift, the rotational levels of HCl exhibit J‐dependent perturbations of 5 to 10 cm−1; the J‐dependent perturbations of the DCl rotational levels lie between 1 and 3 cm−1. Neither hindered rotation nor libration of HCl and DCl in the rare‐gas crystals provides a suitable description of these perturbations. A model suggested by Friedmann and Kimel based on the interaction of the rotational motion with the localized lattice vibrational (``translational'') motion of HCl and DCl at substitutional sites in the rare‐gas crystals is used to interpret the J‐dependent perturbations. A Hamiltonian, which treats the ``translational'' frequencies of HCl and DCl and the strength of the translation—rotation interaction as parameters, is constructed. The Hamiltonian matrix is truncated at a convenient size, and the perturbed energy levels are found by direct diagonalization of the truncated matrix. For a heteronuclear diatomic rotor in a cubic lattice, the translational—rotational states are shown to form a basis for the irreducible representations of a group isomorphous with D 2h . This property allows considerable factorization of the Hamiltonian matrix. The values of the ``translational'' frequencies which give the best fit to the observed rotational spacings are 36 cm−1 in xenon, 57 cm−1 in krypton, and 31 cm−1 in argon. The model in the case of argon is likely to be fuzzy because of the near mass resonance between argon and HCl or DCl. The strength of the translation—rotation interaction as measured by the distance between the center of mass and the center of electrical symmetry in the molecule is 0.13 Å for HCl and 0.096 Å for DCl. A mechanism for the induction of a Q branch in the vibration—rotation spectra of monomeric HCl and DCl in the solid rare gases is discussed in Paper III of this series.