Induced Infrared Absorption of Solutions of H2 and D2 in Liquid Neon

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Abstract
The induced infrared absorption spectra of solutions of H2 and D2 in liquid neon (27.2°K) have been studied. Absorption features were found which differed by less than 7 cm−1 from frequencies of vibration—rotation transitions expected for the freely rotating H2 or D2 molecule. Rotatory energy levels of H2 and D2 in neon solution are, therefore, essentially the same as for gas‐phase molecules. Evidence for quantized translational energy levels for the solute was found. Broad absorption features which shift in frequency with isotope change in solute are assigned to vibration—translation combination bands in which the H2 (or D2) molecule, in addition to increasing its vibrational level, increases its translational level in its solvent cage. The spectroscopic determination of quantized‐translational energy levels permits the estimation of a solute—solvent interaction potential. A harmonic‐oscillator potential to describe the intermolecular interaction is consistent with the observation that the translational energy levels for D2 in neon are √2 smaller than those of H2 in neon. This quadratic potential is also in order‐of‐magnitude agreement with the potential calculated from the Lennard‐Jones and Devonshire cell model of the liquid state. The problems of quantitative determination of translational energy levels from spectroscopic data are discussed. This study has provided a spectroscopic estimate of the liquid‐state interaction potential for the H2 and D2 neon system and an approximate evaluation of all the types of energy of the solute: vibration, rotation, and translation.

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