Collective excitations of thin films of disordered potassium adsorbed on Ag(110)

Collective electronic excitations of disordered potassium adsorbed on Ag(11 0) have been studied by energy loss spectroscopy-low-energy electron diffra ction (ELS-LEED) for overlayer thicknesses varying from 0.3 to 2.3 monolaye rs. We find that, in the long-wavelength limit, the argon surface plasmon e nergy shifts upwards with increasing potassium coverage, contrary to the ex pectation of classical dielectric theory, while its dispersion decreases. T he potassium-induced loss, connected to the 3s to 3p interband transition, sets in above half a monolayer coverage, indicating a filling of the K(3s) state. Above one monolayer coverage collective potassium-induced modes are observed as well. (C) 1999 Elsevier Science B.V. All rights reserved.