INTERATOMIC DISPERSION FORCES AND SMALL-ANGLE SCATTERING FROM RARE-GAS FLUIDS

The structure factor S(q) and the direct correlation function c(q) of rare gases are expected to have a non-analytic \q\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 (AT) interaction. The coefficient c3 of the \q\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 we determine that it will be easier to detect the \q\3 term if the scattering data are analysed iii 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 \q\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 approximately < 0.5 nm-1.