GROWTH AND MORPHOLOGY OF ULTRATHIN METAL FILMS STUDIED BY THE HIGH-RESOLUTION SPECTROSCOPY OF IMAGE STATES THROUGH TWO-PHOTON PHOTOEMISSION

Electrons can be loosely bound to metal surfaces by the attractive image potential. The image states constitute a very sensitive probe of the surface potential and are studied with high resolution by two-photon photoelectron spectroscopy. During the growth of ultrathin metal films, image states from different layers can be distinguished by their local work function. This allows to identify layer-by-layer growth and other growth modes. The changes of the morphology of the films during annealing and the formation of surface al-loys can be monitored. Localization of image-state electrons on small islands results in energy shifts from which the island-size distributions can be de-rived. Localization perpendicular to the surface is observed by the coupling of image states to quantum well states in thin metal films. Because image states are not involved in the metallic or chemical bonding, they provide the perfect test electrons for a sensitive probe of surface properties. 1. Image States The screening of an electron placed at distance z in front of a metal surface leads to a field outside the surface which is identical to the one produced by an opposite charge placed at a distance 2z away (Fig. 1). The attractive image force be-tween the electron and its image charge can be derived from a Coulomb-like potential approaching the vacuum energy Evac for large distances: V (z) = Evac −