Onset of departure from linearized hydrodynamic behaviour in argon gas studied with neutron Brillouin scattering

We have investigated the dynamic structure factor of room-temperature 36Ar at two densities (0.25 and 0.63 times the critical) and at very low momentum transfer (0.3<?<1.3 nm-1). Linearized hydrodynamics agrees with the experimental data only at the higher density and ?<1 nm-1. However, the description in terms of three extended hydrodynamic modes is fully satisfactory at both densities over the whole ? range. We provide the first experimental evidence that the concept of extended hydrodynamic modes is valid for the whole range from hydrodynamics up into the kinetic regime.