Collective Electronic Excitations In Systems Exhibiting Quantum Well States

We present high-resolution electron energy loss spectroscopy (HREELS) measurements on surface plasmon (SP) dispersion in systems exhibiting quantum well states (QWS), i.e.Na/Cu(111),Ag/Cu(111), andAg/Ni(111). Our results demonstrate that the dominant coefficient of SP dispersion for thin and layer-by-layerAgfilms presenting QWS is quadratic even at smallq||, in contrast with previous measurements onAgsemi-infinite media andAgthin films deposited onSi(111). We suggest that this behavior is due to screening effects enhanced by the presence of QWS shifting the position of the centroid of the induced charge less inside the geometrical surface compared withAgsurfaces andAg/Si(111). For ultrathinAgfilms, i.e. two layers, the dispersion was found to be not positive, as theoretically predicted. Annealing of theAgfilm caused an enhancement of the free-electron character of the QWS, thus inducing a negative linear term of the dispersion curve of the SP. Moreover, we report the first experimental evidence of chemical interface damping in thin films forK/Ag/Ni(111). As regardsNa/Cu(111), we found a different dispersion curve compared with thickNafilms, thus confirming the enhanced screening byNaQWS.Results reported here should shed light on the influence of QWS on dynamical screening phenomena in thin films.