Kinetic Theory of Collisional Line Narrowing in Pressurized Hydrogen Gas

Abstract

We measure the incoherent quasielastic scattering of thermal neutrons from hydrogen gas at 85°K and various pressures up to 140 atm. The observed Van Hove self-correlation function (or the self-part of the dynamic structure factor) shows a narrowing effect as the ratio of the observational wavelength to the collisional mean free path increases. The resulting line shape and linewidths are analyzed using an appropriate kinetic equation. The theoretical prediction agrees with experiment when the only parameter in the theory, namely, the self-diffusion coefficient, is properly calculated in the Enskog approximation. We demonstrate experimentally that the linearized Boltzmann equation gives an accurate description of the dynamics of the single-particle motion at moderately high density and in the range of wave vectors and frequencies comparable to inverse collision mean free paths and collision frequencies of the molecules.