Density fluctuations in liquid argon: Coherent dynamic structure factor along the 120-K isotherm obtained by neutron scattering

Abstract

Coherent dynamic structure factors S(k,ω) obtained by means of thermal neutron inelastic scattering are presented. The experiments were performed on liquid ${}_{}{}^{36}{}_{}{}^{}\mathrm{Ar}$ at four densities along the 120-K isotherm covering a range of wave numbers k from 4.2 to 39.0 ${\mathrm{nm}}^{\mathrm{-}1}$. The neutron time-of-flight spectra are corrected for all known experimental effects with an improved data-reduction system. Special attention is paid to corrections for multiple scattering, duty-cycle overlap, and instrumental resolution. The importance of various correction steps is shown. The reliability of the corrected data is assessed by means of two independent consistency checks, viz., the detailed-balance condition and the first frequency moment of S(k,ω). The S(k,ω) data are presented both as a function of k at fixed ω and as a function of ω at fixed k. The peak height and full width at half maximum of S(k,ω) at fixed k are shown for all densities together with the small-k (hydrodynamic) and large-k (free-gas) asymptotes. The frequency moments of S(k,ω), evaluated up to the fourth moment, are consistent with results from computer simulations and from theoretical calculations. The longitudinal current correlation function $C_l$(k,ω), derived from the experimental S(k,ω), is examined and both its peak position (yielding the dispersion curve for longitudinal current fluctuations) and its peak height (a measure of the lifetime of the fluctuations) are discussed.