Growth of silver, gold, and cobalt on Cu(111) studied by high-resolution spectroscopy of image states

Image states are located a few angstroms in front of metal surfaces and constitute a highly sensitive probe of surface properties. With high-resolution two-photon photoemission the image states were measured during the growth of ultrathin epitaxial films of Ag, Au, and Co and Cu(111) at room temperature. For Ag on Cu(111) the image state of the substrate 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 > 100 A diameter. For Au and Cu(111) the image state of the substrate 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 clean copper surface is a clear proof for the growth of three-dimensional islands. For Co and Cu(111) a similar growth mode is found. Annealing experiments reveal a segregation of 1 ML of Cu onto the Co films at temperatures between 100 and 200 deg C depending on the Co coverage.