Revisiting Oxygen Adsorption on Ir(100)

The adsorption and dissociation of O-2 on Ir(100) has been studied with X-ray photoelectron spectroscopy (XPS), low energy electron diffraction (LEED) and scanning tunneling microscopy together with density functional theory calculations. At low coverage, XPS shows a single O is peak at 530.6 eV, associated with bridged O atoms in an Ir(100)-(3 X 1)-O structure. At saturation coverage, an additional peak at 529.5 eV appears, accompanied by an Ir(100)-(2 X 1)-O structure. The O Is peak at 530.6 eV is assigned to oxygen atoms adsorbed on bridge sites of Ir atoms, and the peak at 529.5 eV to oxygen atoms on fourfold hollow sites of Ir. Based on our detailed experimental and computational study, oxygen diffusion into the subsurface region can be ruled out. The present study provides new findings on an additional oxygen state of the O/Ir(100) system. It also provides evidence for the absence of subsurface oxygen in Ir, implying that some discussions on subsurface oxygen in the literature may be misleading.