The secondary electron yield of solid surfaces contains weak but complicated structure, which is greatly enhanced by differentiation with respect to primary electron energy. Auger electron appearance potential features, corresponding to yield changes due to core level excitations, and diffraction features are the major contributions to this structure. The magnitude of the diffraction features decreases with primary energy and depends strongly on temperature and surface order. Surprisingly, even for polycrystalline materials the diffraction features extend out to about 400 eV and completely obscure appearance potential features in this region. We have measured the energy distribution of electrons contributing to each of these kinds of features. The diffraction structure is due almost entirely to elastic electrons while the appearance potential peaks result primarily from electrons with energy less than 30 eV. This difference can be exploited using a double modulation scheme with four-grid LEED optics to separate the two kinds of features. This extends the useful range of Auger electron appearance potential spectroscopy down to energies below 200 eV.