Observation of Excitonic Surface States on MgO—Stark-Model Interpretation

Using the energy and angle-of-incidence dependence of the electron-energy-loss spectra of MgO, we have separated the excitonic transitions from the Mg core levels to the excited states into those of bulk and surface origin. The bulk transitions are very nearly those of the free ${\mathrm{Mg}}^{2+}$ ion. The surface-state transitions can be described by Stark splitting of the energy levels of the surface ${\mathrm{Mg}}^{2+}$ ions in the intense Madelung electric fields at the crystal surface.