Electronic Structure and Kinetics of the Oxidation of Ba and Sr

Photoemission spectra are used to identify unusually narrow chemisorbed levels of ${\mathrm{O}}^{2-}$ on Ba and Sr at 4.6 and 4.8 eV below the Fermi level $E_{\mathrm{F}}$, respectively. Nitrogen on Ba forms narrow states 1.3 and 5.3 eV below $E_{\mathrm{F}}$. Measurements of sticking probability and of electron emission in the dark show that the oxygen-metal chemical activity is initially small, increases to a maximum, and then abruptly decreases as the mass of oxide increases.