A model system for the Lorentz gas can be made [Eder, Chen, and Egelstaff, Proc. Phys. Soc. London 89, 833 (1966); McPherson and Egelstaff, Can. J. Phys. 58, 289 (1980)] by mixing small quantities of hydrogen with an argon host. For neutron-scattering experiments the large H-to-Ar cross section ratio (∼200) makes the argon relatively invisible. Dynamic-structure-factor [S(Q,ω) for ${\mathrm{H}}_2$] measurements at room temperature have been made on this system using the IN4 spectrometer at the Institute Laue Langevin, Grenoble, France. Argon densities between 1.9 and 10.5 atoms/${\mathrm{nm}}^3$ were used for 0.4<Q<5 A${̊}^{\mathrm{-}1}$. Additional measurements were made with a He gas host at densities of 4 and 10.5 atoms/${\mathrm{nm}}^3$; helium is relatively invisible also compared to hydrogen. These experiments are described, and some examples of the results are presented to show the qualitative effects observed. The principle observation is a pronounced narrowing of S(Q,ω) as a function of ω as the argon density is increased. This effect is large at low Q and decreases with increasing Q, and also decreases substantially when helium is used in place of argon. In addition, the shape of S(Q,ω) is more complex than can be accommodated within a simple model, but slightly less complicated than a computer simulation so showing the significance of multiple-collision processes.

• Received 11 September 1989

DOI:https://doi.org/10.1103/PhysRevA.41.1936