Identification of Subsurface Oxygen Species Created during Oxidation of Ru(0001)

The oxidation states formed during low-temperature oxidation (T 5/2 core levels of the two topmost Ru layers. The evolution of the Ru 3d5/2 spectra with oxygen exposure at 475 K and the corresponding O₂ desorption spectra reveal that about 2 ML of oxygen incorporate into the subsurface region, residing between the first and second Ru layer. Our results suggest that the subsurface oxygen binds to the first and second layer Ru atoms, yielding a metastable surface "oxide", which represents the oxidation state of an atomically well ordered Ru(0001) surface under low-temperature oxidation conditions. Accumulation of more than 3 ML of oxygen is possible via defect-promoted penetration below the second layer when the initial Ru(0001) surface is disordered. Despite its higher capacity for oxygen accumulation, also the disordered Ru surface does not show features characteristic for the crystalline RuO₂ islands. Development of lateral heterogeneity in the oxygen concentration is evidenced by the Ru 3d5/2 images and microspot spectra after the onset of oxygen incorporation, which becomes very pronounced when the oxidation is carried out at T > 550 K. This is attributed to facilitated O incorporation and oxide nucleation in microregions with a high density of defects