Quantum Theory of Collision-Induced Absorption in Rare-Gas Mixtures

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Abstract
A theoretical calculation of the collision-induced far-infrared absorption, which has been observed in rare-gas mixtures, is carried out using the quantum-mechanical expression for the absorption coefficient. Using a simple empirical form to describe the variation of the collision-induced dipole moment with internuclear separation, and using plane-wave eigenstates (i.e., assuming straight-line collision paths), an analytic expression is obtained for the spectrum which agrees well with the experiments. Comparison with a previous classical calculation based upon the same model shows that quantum effects are small at room temperature except in the high-frequency wing of the spectrum. A number of analytic integrations allow us to derive simple expressions for the correlation function, relaxation time, static dielectric constant, and other spectral invariants.