Adsorbed xenon atoms as local surface structure probes: The initial growth of thin Ag films on Ru(001)

Photoemission of Adsorbed Xenon (PAX) is shown to provide valuable information on the initial growth mode of a thin, 0.2 monolayer silver film on Ru(001) at 50 K on an atomic scale. By taking advantage of the fact that the Xe(5p_{$\text{3}\text{2}\text{,}\text{1}\text{2}$}) UPS spectra of adsorbed xenon allow a clear distinction between Xe atoms being adsorbed on Ag sites, on Ru sites, and at mixed Ag-Ru sites, a model is proposed, according to which the amount of 0.2 ML silver is distributed on the Ru(001) in the form of small islands without alloying. From the 5p UPS intensities of the various Xe adsorption states the radius as well as the number of these islands (assumed to be circular) could be calculated. The number of these islands agrees remarkably well with the number of defects on the original Ru(001) substrate, which was again “titrated” by means of xenon adsorption. These defects are therefore regarded as nucleation centers for the incipient Ag island formation. This model is supported by accompanying AES, LEED, Xe desorption and work function measurements.