Translational and Rotational Motion of Small Molecules in Liquid Argon

Abstract

Far-infrared studies of dilute of H2, HD, D2, and Ne in argon yield considerable information on molecular motion in this liquid system. Rotational absorption features of H2 and D2 between 200 and 1000 cm−1 indicate that these molecules are free rotors in an argon solution. Since HD is an asymmetric molecule, its rotational and translational motions are coupled. The rotational absorptions for HD, therefore, indicate it is not a free rotor. Absorption features are observed that are assigned to translational transitions of Ne, D2, and H2. The spacings of these bands indicates that the translational energy level is in the range of 20–200 cm−1. A simple cell model that treats the solute (Ne, D2, or H2) as an oscillator trapped in a solvent cavity whose walls consist of Ar atoms is in reasonable agreement with the observed spectroscopic transitions. These studies provide evidence for quantized rotational and translational energy levels in this liquid system. In addition, the HD solution illustrates the effect of rotation-translation coupling in the liquid state.