MULTIPLE SURFACE-PLASMON EXCITATIONS IN OVERLAYERS OF K AND NA ON RU(001)

Electron energy loss spectra in the reflection geometry have been meas ured for potassium and sodium layers on Ru(001) for coverages ranging from N = 1 up to 16-20 layers. The experimental conditions (primary en ergy, angle-of-incidence) allowed excitation of multiple surface plasm ons. For all systems, energy and damping of the surface plasmons toget her with the integral probability for surface plasmon excitation have been determined. For the variation of these parameters with alkali lay er thickness, two coverage regimes can be distinguished that are corre lated to different types of excitations. For N less-than-or-equal-to 1 one-electron excitations are very important; for N greater-than-or-eq ual-to 2 a collective response (well-defined surface plasmon excitatio ns) takes place. The slowly decreasing energy of the surface plasmon w ith increasing alkali thickness to the bulk values is in qualitative a greement with both macroscopic and microscopic descriptions. The thick ness dependences of the damping and the integral excitation probabilit y indicate that one-electron contributions which are very important fo r N = 1 are also present for N = 2 and only slowly decrease with incre asing thickness of the alkali layer. The analysis of the EELS data for N greater-than-or-equal-to 2 shows that the excitation probability fo r multiple surface plasmons follows the Poisson distribution; the data are consistent with the consecutive excitation of individual surface plasmons.