X-ray photoelectron diffraction study of oxygen adsorption on the stepped copper surfaces (410) and (211)

Abstract

We report the first structural investigation of adsorption on stepped metal surfaces using core-level azimuthal X-ray photoelectron diffraction (XPD). The specific systems studied are oxygen adsorption on Cu(410) and Cu(211). Both surfaces show enhanced oxygen adsorption rates compared to the simple low-index surfaces of the terraces, suggesting adsorption sites on the steps. Kinematical LEED analyses verify the step structure of both clean Cu(410) and Cu(211), and also indicate a reconstruction of Cu(211) for exposures ≳ 3 L into a surface with diatomic step heights. At higher exposures of ≳ 75 L, Cu(211) is further observed to facet. Cu(410) is by contrast very stable in structure up to 1200 L exposure. Detailed azimuthal XPD data were obtained for O 1s emission from both surfaces at a low and a high exposure to oxygen. Pronounced diffraction features are seen; these are observed to be sensitive to both the polar angle of emission and the degree of exposure. By comparing the experimental data with single-scattering cluster calculations for different possible adsorption geometries, the most likely adsorption sites for atomic oxygen were deduced. Oxygen is found to lie predominantly on the steps for low exposures of both surfaces: for 0.75 L exposure on Cu(410) it occupies a two-fold site at the top of the step and for 5 L exposure on reconstructed Cu(211), a four-fold site at the bottom of the step. Vertical positions are determined with an estimated accuracy of ± 0.2 Å. At higher exposures ≳ 40 L for the Cu(410) surface, both step and (100) terrace sites are found to be occupied in a modified c(2 × 2) structure. O 1s polar-scan data for Cu(410) also verify that the initial adsorption site is on the step. Such angle-resolved XPS measurements thus have considerable promise for stepped-surface structural studies.