Infrared Induced Absorption of Dilute Solutions of H2 and D2 in Liquid Argon

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Abstract
The induced infrared absorption spectra of dilute solutions of H2 and D2 in liquid argon have been studied using a low‐temperature cell with a path length of 0.5 m. The close correspondence between observed absorptions with narrow band widths (ν½∼20 cm—1) and vibration—rotation transitions calculated using the gas‐phase molecule spectroscopic constants strongly suggests that H2 and D2 rotate freely in liquid argon. An interpretation of the observed broad absorption features (ν½∼100 cm—1) is considered in which they assigned to vibration—translation combination bands involving changes in translational energy of H2 (or D2) in its solvent cage. These translational energy levels shift with isotopic change of the solute. A cell model of the solution system is considered in which the H2 (or D2) molecules are assumed to translate in their solvent cages as isotropic harmonic oscillators. The predicted difference in translational energy levels between H2 and D2 in argon is in satisfactory agreement with the observed isotopic shifts. The selection rules which govern the vibrating—translating solute molecule transitions are derived for the model of the isotropic harmonic oscillator and are consistent with the observed absorptions. An estimate of the translational energy level spacing which is provided by considering the solute in solvent as a particle in a box gives values which are in order of magnitude agreement with the proposed interpretation of the broad absorptions. If the present assignments are correct the results of this investigation provide a basis for the determination of all the types of energy levels of the solute molecule: vibrational, rotational, and translational.