ADSORBATE-INDUCED VARIATION OF THE STRUCTURE AND COMPOSITION OF THE MO0.75RE0.25(100) SURFACE

Clean random alloy surfaces usually exhibit non-bulklike concentration profiles. Adsorption at low temperatures does not modify these profil es as there is thermal activation for interdiffusion of surface atoms required. For strongly chemisorbed species like oxygen or carbon, howe ver, the activation barrier can be overcome prior to desorption leadin g to a new equilibrium distribution of the surface constituents. This paper summarizes the results of different LEED-I(V) studies on the ads orption of hydrogen, oxygen and carbon on the (100) face of the random alloy Mo0.75Re0.25. Hydrogen, which desorbs near room temperature, is found to affect the interlayer spacings only, whilst the concentratio n profile remains completely unchanged. In contrast, enhanced molybden um segregation at temperatures above 600 degrees C is shown to happen with oxygen and carbon adsorption leading to exclusive Mo-C and Mo-O b onds, respectively. The surface structures formed for high coverages d iffer substantially from each other. Oxygen induces a missing row stru cture and is bound in a ''quasi'' three-fold coordinated site. In cont rast, carbon is adsorbed in five-fold hollow sites at the surface and in six-fold coordinated octahedral subsurface sites.