Photoemission studies of the low-temperature reaction of metals and oxygen

X-ray photoemission and synchrotron-radiation photoemission were used to study the reaction of metals and molecular oxygen at low temperatures. Combined core-level and valence-band measurements permit the identification of various oxygen species. When Li, Cs, K, and La are deposited on solid oxygen, an O 1s feature near 535 eV is observed and attributed to the superoxide species. Features ranging from 531.5 to 533 eV are also observed and identified as the peroxide species. In contrast to the case of Li and of most other metal oxides, for Cs or K ``oxides'' an O 1s feature is seen at 528 eV and attributed to the O^2- species in a metallic matrix.