Electron-stimulated desorption studies of O on W(110): Discussion on the excitation mechanism and the binding sites of theO+-yielding species

In the present electron-stimulated desorption studies of O chemisorbed on a flat W(110) surface at room temperature, we present the measurements of the ${\mathrm{O}}^{+}$ ion energy distribution as a function of oxygen coverage and of incident electron energy. The data indicate that the excitation of $\mathrm{W}(4f+5\mathrm{}{p}_{\frac{3}{2}})$ core states is the primary channel for desorbing ${\mathrm{O}}^{+}$ and that the creation of these core holes gives rise to relaxation effects which gradually shift the ${\mathrm{O}}^{+}$ energy distribution towards lower kinetic energy with increasing electron energy. These phenomena indicate that the ${\mathrm{O}}^{+}$-yielding species bind directly to W atoms. We show that these ${\mathrm{O}}^{+}$-yielding species are not the majority atoms which form the $p(2\mathrm{}\times 1)$ structure, but are minority atoms binding at special sites, in agreement with previous reports. We discuss in some detail what these special sites might be, based on our measurements and previous studies. While recognizing the important role of defects and steps in producing ${\mathrm{O}}^{+}$, we suggest that the ${\mathrm{O}}^{+}$ ions observed in our experiments as well as some of previous studies are very likely due to adsorption of oxygen atoms in singly coordinated "atop" sites.