Interatomic dispersion forces and small angle scattering from rare gas fluids

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Abstract
The structure factor S(q) and the direct correlation function c(q) of rare gases are expected to have a nonanalytic mod q mod 3 term at small q. This term is due, in addition to the well known long-range r-6 tail of the two-body interaction, also to the three-body Axilrod-Teller interaction. The coefficient c3 of the mod q mod 3 term of c(q) has a particularly simple state dependence, T-1 in temperature and linear in density, when the AT interaction is present. If no three-body interaction is present C3 is density independent. Therefore the experimental determination of the density dependence of c3 will give unequivocable evidence of three-body forces. On the basis of the integral equation method the authors determine that it will be easier to detect the mod q mod 3 term if the scattering data are analysed in term of c(q) and for measurements at density and temperature not far from the critical point. Under these favourable conditions scattering measurements in the range 1-4 nm-1 will give a precise determination of the mod q mod 3 term. The role of retardation, which changes the r-6 tail into r-7, on the small-q behaviour of S(q) and c(q) is also evaluated and this effect is predicted to be measurable for q-1.