A new concept for the electronic excitations in adsorbed alkali-metal layers is presented. It explains how these excitations evolve with coverage from below one monolayer to thick multilayers, and it establishes a direct relation of these overlayer collective modes to those of clean alkali-metal surfaces.

• Received 10 May 1991

DOI:https://doi.org/10.1103/PhysRevLett.67.2858