OXIDATION OF CESIUM MULTILAYERS

The low temperature oxidation of Cs multilayers was studied using XPS. The Cs 3d5/2 and O 1s core levels were recorded repetitively while leaking oxygen into the experimental chamber, i.e. as a function of oxygen exposure. A negative binding energy shift and an intensity increase are seen for the Cs core level of the oxidized layer. Three O 1s peaks observed on the oxygen-saturated Cs film at 526.45, 530.30 and 533.65 eV are assigned to oxide, peroxide and superoxide ions, respectively. Our results imply that the oxygen species forming first is O2- and O2(2-) followed by O2-. At the onset of superoxide formation, the oxidized film undergoes a change in the electronic properties, probably a metal-insulator transition, that shifts the Cs and O core levels toward lower binding energies by almost 1 eV. This was observed for films with thicknesses ranging from 5 to 20 layers.