Adsorption of oxygen on Mo (112) surface precovered with beryllium: formation of overlayer and electronic properties

Abstract

Adsorption of oxygen on the Mo (112) surface precovered with a pseudomorphic monolayer of beryllium has been investigated at room temperature by AES, LEED and contact potential difference methods. Such a Be/Mo (112) substrate is actually a bimetallic surface where closely-packed atomic Mo ridges alternate with rows of Be atoms. It has been found that at small oxygen exposures (Q < 0.3 Langmuir), the initial sticking coefficient for oxygen S _O on Be/Mo (112) is lower by a factor of ~1/15 than on the clean Mo (112) surface where S _O is close to unity. However, with increasing the oxygen coverage above θ _O ≈ 0.1, the sticking coefficient showed a nonlinear growth, and oxygen saturation of the surface was achieved at Q = 1.6–1.7 L. Oxygen adsorption decreases the work function of the Be/Mo (112) surface and gives rise to appearance of some Auger peaks specific to beryllium oxide, which indicates a change in the chemical nature of the surface. The formation of a polar-covalent BeO compound may be responsible for a self-activation of the surface with respect to oxygen which is reflected in the increase of the sticking coefficient observed under growth of oxygen coverage (a kind of autocatalytic reaction). Annealing of the O/Be/Mo (112) system to T _an  = 1100 K resulted in an additional decrease of the work function and a growth of the ratio between the Auger signals of Be in the oxide and metallic Be adsorbed phases. The presence of BeO molecules was detected up to T _an  = 1600 K, above which they dissociated with desorption of Be.