Experimental Determination of the Long-Range Potential of Argon Pairs by Means of Small-Angle Neutron Diffraction

We have experimentally determined the static structure factor $S(k)$ in low density argon gas for 1.0<k<3.50ex0exnm^-1, by means of the small-angle neutron scattering diffractometer PAXE at Laboratoire L\'eon Brillouin in Saclay. The experiment has been performed at three number densities between 1.51×10²⁷ and 2.30×10²⁷0ex0exm^-3 along the T0ex0ex=0ex0ex138.750ex0exK isotherm. From the data the small-k dependence of the Fourier transform $c(k)$ of the direct correlation function $c(r)$ has been derived and the existence of the |k|³ term in the behavior of $c(k)$ experimentally demonstrated. The experiment has allowed the first direct measurement of the coefficient C₆ of the London dispersion interaction in the pair potential of argon.