ELECTRON LOSS SPECTRA FROM THIN ALKALI FILMS ON AL(111)

Angle resolved inelastic electron scattering has been used to measure the energy and dispersion of the electronic excitations for Na, K, and Cs thin films on Al(111) as a function of thickness. The energy loss spectrum for a two layer film is dominated by a collective excitation similar to that seen for a thick film. This "surface plasmon" mode for a two-layer film exhibits a negative dispersion even larger than that observed for the thick film, even though the damping is appreciably larger for the thin film. The loss spectra in the submonolayer region are fundamentally different, apparently dominated by single particle excitations. The transition from a single particle to a collective excitation occurs quite abruptly as the second layer begins to form.