Molecular dynamics study of intercollisional interference in collision-induced absorption in compressed fluids

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Abstract
When the induced dipole µ(R) is predominantly due to isotropic overlap interaction, the translational collision-induced absorption exhibits a pronounced dip in the spectral density around zero frequency at high densities. This dip is observed in the far-infrared spectrum of rare-gas mixtures and in the Q-branch of the fundamental band of dilute H2 in He. The far-infrared spectrum of Kr in liquid Ar and the fundamental vibrational band of gaseous H2 in He at 300 K, 1150 amagat were obtained by molecular dynamics (MD) calculations with the assumption of pairwise interactions in µ(R) and the potential V(R). Reliable pair induced-dipole and potential models derived from studies of the low-density gas were employed. Although the MD results are in rather poor agreement with the experiment for the Ar–Kr mixture, much better agreement is obtained for the He–H2 mixture. A reason for this results is advanced. The destructive interference of µ(R) in successive collisions and the resulting large cancellation of spectral intensity makes this phenomenon a particularly sensitive probe of molecular interactions. The conceivable role of many-body effects associated with irreducible three-body terms in the dipole moment and potential energy is discussed.

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