Interaction of oxygen with Pd(111): High effective O₂ pressure conditions by using nitrogen dioxide

High effective O₂ pressures can be simulated by using NO₂ as a source of atomic oxygen on Pd(111) at exposure temperatures at or above 530 K. Large oxygen concentrations (θ_O=0.0-3.1 ML) have been studied using TPD, AES, HREELS, and XPS. TPD results show four O₂ desorption peaks at 625, 715, 765, and 800 K. The low temperature desorption states have never been reported in previous work concerning oxygen adsorption on Pd(111). Upon heating an atomic oxygen adlayer, diffusion of oxygen into the near surface region is competitive with O₂ desorption for θ_O=0.76-1.4 ML. For θ_O > 1.4 ML, spectroscopic evidence shows the presence of an oxide on or near the surface. Our results for the interaction of atomic oxygen with Pd(111) are compared with the behavior of atomic hydrogen on Pd(111). Finally, a model for the chemical state and desorption behavior of oxygen for these coverages is proposed.