ELEMENTARY EXCITATIONS IN ALKALI-METAL OVERLAYERS PROBED BY ELECTRON-ENERGY-LOSS SPECTROSCOPY - QUANTUM-MECHANICAL EFFECTS

The spectrum of electronic excitations of Na and K overlayers adsorbed on Al(111) has been studied by means of electron-energy-loss-spectroscopy experiments, and self-consistent, dynamical-response calculations. Losses are observed in two distinct energy and coverage domains. The loss observed during the formation of the second atomic layer shows novel features, which are traced to the intrinsic quantum-mechanical nature of the excitations being probed, and their spatial confinement by the overlayer.