Induced Rotation—Translation Spectra of Solutions of H2, HD, and D2 in Liquid Argon

Abstract

Dilute solutions of H2, D2, and HD in liquid argon are studied by examining the 1000–140-cm−1 region of their induced absorption spectra. Since the absorption features overlap extensively, an empirical profile analysis is performed. For H2 and D2 several relatively sharp absorptions (Δν1/2≈30 cm−1), which have a Lorentzian line shape and closely correspond to the expected transitions of the freely rotating solute molecules, are observed. The width of these lines appears to be due to uncertainty broadening which depends on the time the absorbing solute molecule spends in a particular solvent cavity. Other features, with a 150-cm−1 half-width, are fitted with a four-parameter exponential equation and are believed to involve quantized translational energy levels of the solute. The range of translational energy levels, which is determined by the distribution of solvent cavity sizes, is responsible for their increased bandwidths. The HD spectrum is unusual in that the rotational selection rules are relaxed and transitions corresponding to ΔJ = +2, +3, and +4 are observed. Furthermore, all of the rotational absorption features are anomalously broad (Δν1/2≈50 cm−1) and their maxima are shifted from the gas-phase frequencies in an irregular pattern. Because of its asymmetrical mass distribution, rotational motion of HD is accompanied by a translation of the molecule in its solvent cavity. This rotation—translation interaction has been given quantitative form in a perturbation treatment by Friedmann and Kimel, and the results are consistent with the observed selection rules, frequency shifts, and linewidths in the spectrum of HD in liquid argon.