Plasmon dispersion and damping in two-dimensional electron gases on metal substrates

Herein we report on high-resolution electron energy loss spectroscopy (HREELS) measurements on surface plasmon dispersion in systems exhibiting quantum well states, i.e. Na/Cu(111), Ag/Cu(111), and Ag/Ni(111). Our results demonstrate that the dominant coefficient of surface plasmon dispersion for thin and layer-by-layer Ag films presenting quantum well states is quadratic even at small q, in contrast with previous measurements on Ag semi-infinite media and Ag thin films deposited on Si(111). We suggest that this behavior is due to screening effects enhanced by the presence of quantum well states shifting the position of the centroid of the induced charge less inside the geometrical surface compared to Ag surfaces and Ag/Si(111). For ultrathin Ag films, i.e. two layers, the dispersion was found to be not positive, as theoretically predicted. Annealing of the Ag film caused an enhancement of the free-electron character of the quantum well states, thus inducing a negative linear term of the dispersion curve of the surface plasmon. Moreover, we report the first experimental evidence of chemical interface damping in thin films for K/Ag/Ni(111). As regards Na/Cu(111), we found a different dispersion curve compared to thick Na films, thus confirming the enhanced screening by Na quantum well states. Results reported here should shed light on the influence of quantum well states on dynamical screening phenomena in thin films. © 2011 by Nova Science Publishers, Inc. All rights reserved.