The early stages in the oxidation of Na

We report soft x‐ray photoemission results on the early stages of the oxidation of Na at ∼80 K. The first stage (up to ≳0.5 L exposure) is characterized by a narrow O 2p level and a Na 2p surface core level shifted by −0.55 eV with respect to the bulk core level. The Na 2p surface plasmon loss intensity shifts toward smaller energies, consistent with a decrease in surface conduction electron density. The second stage (starting from ∼1 L) is characterized by the emergence of ‘‘di‐oxygen’’ valence levels and a Na 2p component shifted by ∼0.8 eV with respect to the bulk binding energy. The oxidation is strongly nonuniform, as suggested by the small oxygen doses needed for the onset of the second oxidation stage, and the continued presence of the bulk plasmon loss from metallic Na. The two new Na 2p binding energies are tentatively attributed to different final‐state screening of metallic Na adjacent to dispersed oxygen atoms in the first stage, and of the Na atoms with mostly oxygen nearest neighbors in the second stage. Observations regarding large negative core level shifts upon large oxygen exposures are also summarized.