Schematics along a vertical scale of energy of the origin and nature of the different excitations created in the electron population of the solid MgO by the high energy incident electrons. The parts of the spectrum selected in the different colour frames have been extracted from the energy loss spectrum shown in Figure 9: (i)bottom: individual atomic electrons excitations from core-loss levels, respectively O-K and Mg-K; (ii) collective (Pl) and individual interband transitions in the valence and conduction electron gas.