Current Problems in the Quasi-elastic Incoherent Neutron Scattering and the Collective Drift of Molecules

Abstract

The determination of the self-diffusion coefficient \( D_{\text{s}} \) is one of well known applications of the quasi-elastic incoherent neutron scattering. Here we will show that the half-width of the neutron peak considered as a function of wave vector can be used for the determination of (1) the residence time \( \tau_{0} \) for water molecules and (2) the very important ratio \( D_{\text{c}} /D_\text{s} \) where \( D_{\text{c}} \) is the collective part of the self-diffusion coefficient, caused by its drift in the field of thermal hydrodynamic fluctuations. The applicability region for the simplest diffusion approximation is discussed in details. The influence of the rotational motion of water molecules on spectra of the intermediate scattering function (ISF) is studied. A new type of the high-frequency asymptote for the ISF-spectra is predicted.