The hydrodynamic limit for dense nitrogen and argon gases

The behaviour of the coherent scattering functions S(Q, omega ) (where h(cross)Q and h(cross) omega are the momentum and energy transferred in radiation scattering experiments) in the limit of small Q approaching the hydrodynamic limit has been discussed in many theoretical papers. However, experimental results have mainly been confined to liquid metals and near-quantum fluids. This paper describes the observed behaviour of S(Q, omega ) as Q is increased away from the hydrodynamic region for a dense gas sample at constant density. The data are compared to theoretical predictions, for which there are essentially no adjustable parameters, and the region where the theory is successful as well as the initial departures from it are identified.