GROWTH OF AG, AN, AND CO ON CU(111) STUDIED BY HIGH-RESOLUTION SPECTROSCOPY OF IMAGE STATES

Image states are located a few angstroms in front of metal surfaces an d constitute a highly sensitive probe of surface properties. With high -resolution two-photon photoemission the image states were measured du ring the growth of ultrathin epitaxial films of Ag, Au, and Co on Cu(1 11) at room temperature. For Ag on Cu(111) the image state of the subs trate is observed up to the coverage of one monolayer (ML) as expected for a layer-by-layer growth. The observation of an unshifted Ag image state in the submonolayer regime indicates Ag islands of more than 10 0 Angstrom diameter. For Au on Cu(111) the image state of the substrat e is observed for coverages up to 1.7 ML. The simultaneous observation of image states from 1 and 2 ML high islands as well as from the clea n copper surface is a clear proof for the growth of three-dimensional islands. For Co on Cu(111) a similar growth mode is found. Annealing e xperiments reveal a segregation of 1 ML of Cu onto the Co films at tem peratures between 100 and 200 degrees C depending on the Co coverage.