Spectroscopic Evidence for Rotation—Translation Coupling in the Liquid State: Infrared Absorption of HD in Liquid Argon

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Abstract
The fundamental vibrational spectrum of HD in liquid argon has been studied. Features were observed which correspond approximately to vibration—rotation transitions of an isolated gas‐phase molecule, indicating that HD rotates in this system. Other features in the spectrum have been assigned to vibration—translation combination bands, providing evidence for the existence of quantized translational energy levels for HD in its solvent cavity. The spectrum is unusual in that the rotational selection rules are relaxed and transitions corresponding to ΔJ=0, ±1, +2, +3, and +4 are present. Furthermore all the vibration—rotation absorption features are broad 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 by a perturbation treatment by H. Friedmann and S. Kimel, and these results are consistent with the observed selection rules, frequency shifts, and linewidths in the spectrum of HD in liquid argon.