The various effects that can occur when the primary electron energy, $E_p$, is varied in a reflection-energy-loss experiment are considered. For $E_p\ge 100$ eV, the dominant effect is an increase of the electron mean free path with increasing $E_p$. By performing energy-loss measurements with $100 \mathrm{eV}\le E_p\le 2000 \mathrm{eV}$, it is possible to unambiguously separate bulk and surface features in loss spectra. That technique has been applied to the MgO (100) surface, and both Mg intraionic and O-to-Mg interionic transitions have been studied. The former transitions are found to agree well with the excited-state spectra of free ${\mathrm{Mg}}^{2+}$ ions, while the latter agree more closely with itinerant-electron calculations of MgO. Intrinsic surface-state transitions are seen in both ${\mathrm{Mg}}^{2+}$ and O-to-Mg spectra. The Mg core-level surface-state spectra can be explained by Stark splitting of the surface ${\mathrm{Mg}}^{2+}$ levels in the intense electric fields at the crystal surface. The surface-state structure seen in O-to-Mg loss spectra agrees with discrete variational $X\alpha$ calculations of the MgO (100) surface and disagrees with linear combination of atomic orbitals calculations of the same surface. A low-energy-loss peak, possibly associated with surface defects, is seen on some surfaces; the exact nature of the surface defects involved is not yet clear.

• Received 23 May 1980

DOI:https://doi.org/10.1103/PhysRevB.22.4764