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Abstract:
Ru(001) surfaces covered by different amounts of Cs (θCs=0.08, 0.25 and 0.33 (to the monolayer)) are exposed to O2 and analyzed with high-resolution electron energy loss spectroscopy. For θCs = 0.25 and 0.33 a superoxo (O2−) species can be stabilized at 90 K if the metallic charge in the Cs overlayer is first withdrawn in an oxidation step leading to chernisorption of atomic oxygen and the disappearance of an interface plasmon. O2− is characterized by its stretching vibration at 140.3 meV, overtones of it at 278.4, 414.0 and 546 meV and the O2−-against-surface vibration at 15 meV. From the overtone energies the dissociation energy of O2− is derived as D = 3.9 ± 0.3 eV close to the gas-phase value. The O2− vibrations are excited via a negative-ion resonance at 5.0 ± 0.3 eV above the vacuum level. The scattered electron intensity has its maximum at 35° with respect to the surface normal consistent with the assumption that the 4σEu orbital is involved in the negative-ion resonance. Arguments are given that O2− is oriented with its axis parallel to the surface.
