The structure and kinetics at, and close to, interfaces is of importance in many sustainable-energy devices (e. g. electrolytes and electrodes, see Chap. 7), but these properties are difficult to establish. Within certain constraints, the neutron reflection (NR) experiment can establish the scattering characteristics beneath a surface by measuring the reflected intensity as a function of angle. Above a critical angle (representing Q), total reflection occurs, but below this each layer interface pro­duces an oscillating reflected amplitude with period AQ = 2n/T, where T is the thickness of the layer. However, the measured reflected intensity is the total from all interfaces present, and because phase information is lost, the usual way forward is
to fit the measured signal with models. In practice it is the variation of sample composition within the depth of the sample that is of interest, and this is charac­terized as the scattering-length density (SLD) profile, which is the sum over the number density of each isotope at a given depth, z, times its bound coherent neutron-scattering length. The essential advantages of neutron measurement of reflection is the variation of scattering length with isotopic nuclei, which allows contrast variations, measurement of buried layers, and favours the light elements in the presence of heavier ones found in energy materials.

The main constraints in NR are the comparatively large and atomically-flat surface that is required, and establishing suitable models for analyzing the results.