Metal-insulator transition during oxidation of cesium films

Article abstract of DOI:10.1016/0039-6028(92)90690-8

The oxidation of Cs films in the thickness range of 5 to 25 layers was investigated at 95 K using UPS and HREELS combined in situ. The whole film undergoes a metal to insulator transition at an oxygen dose equivalent to the formation of Cs₂O, the normal oxide. This state is characterized through a single, somewhat broadened 2p line of O^2- in UPS, the opening of a gap around E_F (vanishing electronic losses as seen through HREELS), and the appearance of a strong Fuchs-Kliewer phonon intensity. Below this transition, i.e., in the range of suboxides, the Cs surface plasmon is observed with its energy shifting to small values as the metallic electron density gets diluted with O₂ dose. Above the transition, the UP spectra change into multiplet spectra well known for Cs dioxygen species. The oxidation process can be described by a Cs → suboxides → Cs₂O → peroxides sequence.