Atomic Motion in Liquid Argon

The atomic motion in liquid argon at 94°K has been investigated by inelastic scattering of 4.1Å neutrons. Special attention was paid to the collective aspects of the motion, and the observations were therefore confined to momentum transfers $\kappa$ in the region $1.3<\mathrm{}\kappa <3.3\mathrm{}$ ${\mathrm{\AA }}^{-1\mathrm{}}$, which includes the main peak of the liquid structure factor. The results are compared with various models. These comparisons give strong evidence of collective modes, and an extreme polycrystalline model, in which even the polarization of the phonons is retained, gives the best description of the data. There is also definite evidence of a second Brillouin zone in liquid argon. A dispersion relation is obtained with a slope close to the measured velocity of sound and in good agreement with earlier neutron results. The frequency spectrum has been determined and is compared with the results of a computer calculation by Rahman.