A633

Surface Science 259 (1991) 288-300 North-Holland

Sulfur adatom diffusion on the Cu(111) surface B.J. Hinch a, J.W.M. Frenken b, G. Zhang c and J.P. Toennies c a Department of Chemistry and Laboratory for Surface Modification, Rutgers University, Piscataway, NJ 08855-0939, USA b FOM-lnstitute for Atomic and Molecular Physics, Kruislaan 407, 1098 SJ Amsterdam, Netherlands c Max-Planck-lnstitut f~r Stri~mungsforschung, Bunsenstrasse 10, D-3400 G~ttingen, Germany Received 5 April 1991; accepted for publication 1 July 1991

This paper presents the first low-energy, quasi-elastic He-atom scattering study of adsorbate atom diffusion. At 820 K sulfur adatoms segregated onto the close-packed (111) surface of a copper crystal at a rate ~ 0.01 ML/h. These adatoms produce significant diffuse intensity of scattered He and, as evident from the quasielastic energy broadening, are highly mobile. The diffusion coefficient (D ffi 2.9 × 10 -5 cm 2 s - I ) exhibits no surface coverage dependence, indicating relatively weak sulfur-sulfur interactions on the Cu( l l l ) substrate.

Surface Science 259 (1991) 301-313 North-Holland

Passivation and promotion of oxidation: reaction of oxygen with Fe/GaAs(100) Nan Li, Ge Meng, Kan Xie and Zhang-da Lin Laboratory for Surface Physics, Institute of Physics, Chinese Academy of Sciences, Beijing, 100080, People's Republic of China

Received 22 March 1991; accepted for publication 25 June 1991

Coadsorption of oxygen with Fe in a few monolayer coverages on GaAs(100) surfaces has been studied with XPS and UPS. The experimental results reveal several interesting phenomena in oxidation of the Fe/GaAs(100) systems, which may include oxidation passivation for the Fe overlayers, oxidation promotion for the substrate species, saturation of the oxidation and oxygen adsorption as well as out-diffusion of cations on the surface of the substrate. An interpretation based on the Fe overlayer induced changes in surface bonding states is proposed and another mechanism in relation to the catalytic oxidizing activity of Fe 3+ is discussed for explaining the oxidation promotion effect for the Fe/GaAs(100) systems.

Surface Science 259 (1991) 314-322 North-Holland

Anisotropic surface roughening: {110} faces of fcc metals H.P. Bonzel, U. Breuer Institut flit Grenzfliichenf orschung und Vakuumphysik, Forschungszentrum Jiilich, Post f ach 1913, D-5170 Jiilich, Germany

and

Michael Wortis Physics Department, Simon Fraser University, Burnaby, Canada BC V5A 1S6 Received 21 February 1991; accepted for publication 4 July 1991

{110} oriented surfaces of fcc metals are structurally anisotropic and, for this reason, may be expected to roughen in an anisotropic manner. The microstructure of such surfaces may continue to exhibit pronounced anisotropy up to and, indeed, beyond the roughening temperature. This anisotropy shows up at a macroscopic level as anisotropy in the facet shape of small particles and at the microscopic level as anisotropy in the step density. In particular, the proliferation of (110) and (001) oriented steps should become noticeable at different characteristic temperatures below TR of the {110} face. Various arguments and recent experimental data for Pb{110} are quoted in support of this general rule.