Quasielastic neutron scattering (QENS)

 

Quasielastic neutron scattering (QENS) is a neutron-scattering technique, which provides access to low-energy collective motions, diffusional motions or molecular reorientations. Hence, QENS is often employed to study the dynamics of proteins, polymers or in ionic liquids. Yet, it can also probe the diffusion in solids, for instance the hydrogen diffusion in metals and hydrogen storage materials or in solid ion conductors. Furthermore, magnetic fluctuations can be investigated.

QENS probes the energetically broadened signal around the elastic line and measures energy transfers down to the sub-µeV-regime. Hereby, dynamic processes on time scales from 10-13 to 10-7 s and on length scales from 1 to 500 Å can be studied. From the quasielastic signal, we can determine the type of diffusion, i.e. rotational or translational diffusion, as well as activation energies of involved processes. Since QENS makes use of the incoherent scattering cross sections, the behavior of hydrogen-containing molecules or groups is readily measurable.

  QENS scheme Copyright: © MZ

The figure displays schematically the signal of an elastic line (blue curve) at a temperature T1, at which no dynamic processes occur. By increasing the temperature of the sample to T2 > T1, a dynamic process is excited, for instance the diffusion of a molecule on a surface. This diffusional process produces a quasielastic signal, which is the broadened contribution (red curve).

At our institute we employ QENS in order to study the diffusional modes of water molecules and organic molecules at surfaces of nanoparticles, see QENS project.

Further literature on QENS:

A practical guide to quasi-elastic neutron scattering, Mark T. F. Telling, 2020, Royal Society of Chemistry

Quasielastic Neutron Scattering and Solid State Diffusion (Oxford Series on Neutron Scattering in Condensed Matter, 13, Band 13), Oxford University Press; New Edition (30. November 2000)