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

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.