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Surface Science 241 (1991) 1-5 1 North-Holland

Equilibrium of segregation in Ag/Cu(111)" kinetics and isotherms J. Eug6ne 1, B. Aufray and F. Caban6 Laboratoire de Mdtallurgie, URA 443-CNRS, Facult~ des Sciences et Techniques Saint JerOme, Av. Escadrille Normandie-Niemen, 13397 Marseille, France

Received 18 July 1990; accepted for publication 25 September 1990

In this study, dissolution and segregation kinetics were used to deduce isotherms with which equilibria of segregation were investigated. By using this method on a model system Cu(Ag) we observed a phase transition of Ag on the Cu surface at 450 o C. This phase transition has a critical point at half-coverage: below this point the two isotherms obtained from dissolution and segregation kinetics are similar to each other but above 50% coverage the two isotherms diverge. This can be interpreted either by a "percolation transition" or by a decrease of relaxation energy related to increases of silver surface concentration.

6 Surface Science 241 ( 1991 ) 6-10 North-Holland

Electron-stimulated desorption of lithium from LiF and the influence of metal islands on the surface J. Sarnthein, P. Wurz, W. Husinsky and G. Betz Institut f ~ Allgemeine Physik, Technische Universitiit Wien, Wiedner Hauptstrasse 8-10, A-1040 Wien, Austria

Received 2 January 1990; accepted for publication 22 August 1990

Our experiments focus on the metal overlayer on LiF created by bombardment with low energy electrons. At higher temperatures the metal atoms did not form a continuous film but three-dimensional islands. This was derived from measurements of the activation energy for desorption that varies as a function of island size (Kelvin model). We also found that island sizes rise with current density. These results were obtained using electron stimulated desorption and thermal desorption techniques.

Surface Science 241 (1991) 11-24 11 North-Holland

Influence of coadsorbed bismuth and copper on carbon monoxide adlayer structures at ordered low-index platinum-aqueous interfaces Si-Chung Chang and Michael J. Weaver Department of Chemistry, Purdue University, West Lafayette, IN 47907, USA

Received 25 June 1990; accepted for publication 15 August 1990

The influence of predosed bismuth and underpotential deposited (UPD) copper on the adlayer structure of carbon monoxide on ordered Pt(ll l) , (100), and (110) electrodes in 0.1 M HCIO 4 has been examined by means of in situ FTIR spectroscopy in the C-O stretching 0'co) region. The irreversibly adsorbed and electroactive nature of the bismuth adlayers enabled the effect to be examined of known and adjustable metal coverages, 0hi, upon the Pet spectra for various dosed CO coverages, 0co. On Pt(ll l) , increasing 0Bi yields approximately commensurate decreases in the saturation CO coverage, along with a progressive shift in the CO site occupancy from twofold bridging to terminal coordination as deduced from the relative Pco band intensities. At the "saturation" bismuth coverage, 0Bi(sat ) = 0.21, the CO binding is exclusively terminal throughout the 0co range, 0.05 S 0co < 0.37. These results are consistent with adsorption of bismuth chiefly in bridging sites, yielding a microscopically intermixed adlayer with CO. A similar effect of increasing 0hi upon 0co [up to 0re(sat ) = 0.43] is seen on Pt(100); the bridging ~¢o feature is again preferentially, yet incompletely, removed by 0Bi(sat ). These findings indicate that an intermixed layer is formed on Pt(100) with bismuth again present at twofold bridging sites. By contrast, saturated CO adlayers in the presence of UPD copper on Pt(111) form segregated domains as