Initial stage of O2 adsorption on Ag(100): vacancies and local oxide formation.

The early stage of molecular oxygen adsorption on the Ag(100) surface was reexamined with scanning tunneling microscopy (STM) and density functional theory. Using an oxygen-functionalized STM tip, we demonstrated that the simple interpretation of the individual objects randomly distributed over the surface as chemisorbed oxygen atoms is incompatible with the cross-shaped structures observed in atomic-resolution images in the current work. In our model, each feature is an oxide-like cross formed by two intersecting O-Ag-O-Ag-O chains, with oxygen atoms in vacancies in the Ag(100) layer. Furthermore, we found numerous oxygen-free vacancies randomly diffusing over the surface under the STM probe even at 77 K.