Electronic excitations on metal surfaces and nano-structures

Abstract

Collective electron excitations on a free-electron-like metal surface like Al have been studied by X-ray photoemission spectroscopy. In this review, we discuss photoemission studies on quantization of electron states due to confinement in nano-structures like epitaxial Na films on Al(111). The electronic structure of Ar nanobubbles embedded in the sub-surface region of Al has been discussed. For Al, an asymmetric line-shape is observed for both monopole surface and bulk plasmon in good agreement with theory. The relative contributions of the intrinsic, extrinsic and the interference processes to the surface plasmon intensity are determined from theoretical plasmon line-shape calcu lations and angle-dependent photoemission. The characteristics of the multipole plasmon mode are also discussed. Using angle resolved photoemission, Na thin films on Al(111) have been studied for different thicknesses. We find features in the valence band spectra that behave like quantum well resonances in a narrow photon energy range where the overlayer collective excitations are observed. These resonances are observed because of quantum confinement due to a potential step at the Na/Al interface and the dynamical enhancement of the electric field in the overlayer. In case of argon nano-bubbles in Al, we find that the Ar 2p binding energy and the Doniach-Sunjic asymmetry of the core level line shape vary systematically as functions of Ar⁺ implantation energy and number of ions bombarded (fluence). These observations are explained by relating the strength of Al conduction electron screening of the core -hole created in the photoemission final state to the size of the Ar nano-bubbles.