Theoretical investigation of the dip in the far infrared absorption spectrum of dense rare gas mixtures

Abstract

The far infrared absorption spectrum of dense rare gas mixtures is analyzed in terms of two-, three-, and four-body contributions. It is argued that pair, triplet, and quadruplet terms must be accurately known at short times, in order to describe the overall spectral shape, since the long time region, where diffusion processes can occur, is drastically reduced by the range of the interaction and the cancellation effect. A model calculation is presented where each piece of the total correlation function C(t) is expressed as a Gram–Charlier series. The parameters are then estimated with the help of a lattice gas model which includes a correction to the Kirkwood superposition approximation in the case of three-body correlations. The dip in the reduced spectral density is interpreted as a dynamical cancellation effect between two-, three-, and four-body correlation functions, the role played by the cage effect at liquid density being exacerbated by the cancellation. The width and the depth of the dip are found to be strongly dependent on the mass ratio, density, and concentrations, and some comparisons with experimental data are made for Ne–Ar and Kr–Ar liquid mixtures. Finally, the phenomenological interpretation of the dip given by Van Kranendonk and Lewis in the 1970’s (intercollisional interference effect at low density) is discussed in detail and the possible relationship with the present theory is examined.