Observation of the Plasmon Excitation in a Cs Overlayer on Si(100) 2×1

Angle-integrated low-energy electron-energy-loss spectroscopy (EELS) was measured as a function of coverage for studying electronic excitations in Cs atoms deposited onto a clean Si(100) 2×1. surface and simultaneously AES, LEED, Δ φ, and UPS were observed. The energy loss peak (1 eV) in the EEL spectrum appears with large intensity at the saturation coverage of a half monolayer and is not observed at lower coverages. This energy loss is assigned to the plasmon excitation in Cs overlayer on silicon as observed in alkali atoms on noble and transition metals. It is suggested that the observed plasmon excitation in Cs atoms has a one-dimensional character from the standpoint of the geometrical model proposed by Levine [J. D. Levine: Surf. Sci. 34 (1973) 901], where Cs atoms form one-dimensional chains on the raised site at a half monolayer.