Oxygen adsorption on the Ru(101¯0) surface: Anomalous coverage dependence

Oxygen adsorption on to Ru(101¯0) results in the formation of two ordered overlayers, i.e., a $c(2\mathrm{}\times 4)-2\mathrm{O}$ and a $(2\mathrm{}\times 1)p2mg-2\mathrm{O}$ phase, which were analyzed by low-energy electron diffraction (LEED) and density functional theory (DFT) calculation. In addition, the vibrational properties of these overlayers were studied by high-resolution electron loss spectroscopy. In both phases, oxygen occupies the threefold coordinated hcp site along the densely packed rows on an otherwise unreconstructed surface. The O atoms are attached to two atoms in the first Ru layer Ru(1) and to one Ru atom in the second layer Ru(2), forming zigzag chains along the troughs. While in the low-coverage $c(2\mathrm{}\times 4)-\mathrm{O}$ phase, the bond lengths of O to Ru(1) and Ru(2) are 2.08 and 2.03 Å, respectively, corresponding bond lengths in the high-coverage $(2\mathrm{}\times 1)-p2mg-2\mathrm{O}$ phase are 2.01 and 2.04 Å (LEED). Although the adsorption energy decreases by 220 meV with O coverage (DFT calculations), we observe experimentally a shortening of the Ru(1)-O bond length with O coverage. The $v(\mathrm{R}\mathrm{u}-\mathrm{O})$ stretch mode is found at 67 meV $\left[c\right(2\mathrm{}\times 4)-2\mathrm{O}]$ and 64 meV $\left[\right(2\mathrm{}\times 1)p2mg-2\mathrm{O}].$