Is spin-echo resolved grazing incident scattering (SERGIS) useful to study dynamics at surfaces and interfaces?

Surface excitations with energy e and momentum q parallel to the surface can in principle be detected by neutron scattering. The issue is if spin-echo methods, widely used to detect low energy excitations in bulk, can also be used for detecting surface excitations. This issue comes at a time when spin-echo methods are proposed to resolve one of the components of neutron momentum transfer in near surface scattering (SERGIS). The conclusion is that no surface excitation can be measured with momentum perpendicular to the plane of specular reflection, i.e. when q x  = 0. If q x  ≠ 0, the observation of surface excitations is better carried out in an experiment in which spin-echo is used to resolve energy exchange for the measurement of the momentum transfer.