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The width of neutron spectra is studied on the basis of kinetic theory as a function of wavenumber and density and appears to be determined by the heat mode of the fluid. For high-density fluids an interference of static and dynamic effects in this mode is observed.In two previous publications, the dominant eigenmodes of a hard-sphere fluid were approximately determined on the basis of kinetic theory [1,2]. Thus, the extensions of the five hydrodynamic modes, as well as the self-diffusion mode to large wavenumbers were included in these calculations. It appeared that like in hydrodynamics the (extended) heat mode determined both the width and the height of the neutron spectrum S(k, ~o) as a function of frequency 6o for a large range of wavenumbers k. In particular, it was shown that the so-called de Gennes narrowing of S(k, co) at high densities is due to a considerable softening of the heat mode eigenvalue zh(k ) which becomes diffusion.like at about ko ~ 21r where o is the hard-sphere diameter. A comparison with neutron scattering data for liquid argon [3] and rubidium [4] showed that the heat mode also dominates S(k, co) for these fluids.In this letter, we report new results for Zh(k), which were computed from the generalized Enskog theory for a much wider range of densities and wave. numbers and in a much higher approximation than before. We will compare these results with recent molecular dynamics (MD) data for hard spheres [5,6] as I Permanent address: The Rockefeller University,

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The lowest lying eigenmodes of a classical fluid have been approximately demrmined for a wide range o~" ¢~et:sities and w~venumbers. The most important eigcnmodes arc dlrcel extensions ot the th:roe hydt'odynamie heat and sotmd modes to much larger wavenuml0ors. A new and consistent imcrpr~tation of neutron spectra and relatea molecn.lar dynamics simulations in terms of these modes is made. AL~o experimental predletio~s ar~ disett~sed, so~e of whleh seev~ laarticularly suitable f~r investigating with sp;'llalion sources.

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