Thin Ag/Cu‐alloy films on a Ru(0001) substrate were investigated with respect to the two‐dimensional miscibility properties of the two component metals using thermal desorption spectroscopy (TDS), low‐energy electron diffraction spectroscopy (LEED), Auger‐electron spectroscopy, angle resolved ultraviolet photoemission spectroscopy (ARUPS), and Xe adsorption. The desorption of pure Ag and pure Cu submonolayers from Ru(0001) proceeds with zeroth order kinetics around 1000 and 1200 K, respectively. The Ag‐TD spectra from the alloy films show a change from zeroth to first order kinetics as the Cu concentration [ΘAgCu≤1 ML (monolayer)] increases between 0≤ΘCu≤0.5 ML. TD spectra of adsorbed Xe, in which one clearly can distinguish between submonolayer islands of pure Ag and pure Cu on Ru do not exhibit a distinct heterogeneity in the case of Ag/Cu‐alloy films. Instead the desorption peaks shift continuously depending on film composition. Furthermore no Ag induced LEED superstructure typical for pure Ag on Ru(0001) is observed if ΘCu≤0.07 ML is coadsorbed. All the results indicate that Ag is diluted in a Cu matrix, in contrast to the 3D alloy system. Photoemission of adsorbed xenon (PAX) data suggest the formation of densely packed two‐dimensional Ag/Cu islands on the Ru(0001) surface. The dispersion of the electronic Ag(4d)‐ and Cu(3d)‐valence bands as derived from ARUPS measurements decreases with decreasing concentration of the respective component again suggesting dilution and increasing interatomic separation between alike atoms. Monte Carlo simulations of the measured Ag‐TD spectra yield the lateral interaction parameters, which allows to determine the critical temperature TC, where separation in the Ag/Cu layers should take place. It turns out that TC≤198 K, in agreement with our experimental results.

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