6 Coherent quasielastic neutron scattering

Abstract

Abstract QENS on diffusing atoms with a sizable coherent neutron scattering cross section yields information on interference effects and correlated motion contained in the coherent scattering function S(Q, ω). As an introduction to this complex situation, this chapter begins by describing first coherent elastic scattering due to short-range order. The structure factor SSROM(Q) for short-range order of a lattice fluid can be expressed in terms of Cowley's short-range order parameters. For a diffusing lattice fluid, coherent QENS consists of a single Lorentzian: its intensity is given by SSRO and its width Λ and thus the collective diffusion coefficient Dc exhibit the so-called de Gennes narrowing: at the Q value where SSRO has a maximum, Λ and Dc exhibit a minimum. Furthermore, elastic distortion scattering (Huang scattering) and quasielastic scattering on distortions combined with diffusion is dealt with.