Interaction of oxygen with sodium at 80 and 20 K

We have studied the interaction of oxygen with evaporated Na films at 80 and 20 K, by means of core-level and valence photoemission spectroscopy and photoexcited Auger spectroscopy, as well as plasmon loss structures associated with core-level emission. The initial oxidation at 80 and 20 K is characterized by an oxygen 2p resonance at about -3.4 eV with respect to the Fermi level, increasing intensity in the Na-O interatomic Auger transition, increasing width and intensity of the Na 2p core level, and changes in the surface-plasmon loss region which might be interpreted as a decreasing ``free-electron'' density. Further oxidation leads to oxygen valence emission at -7.4 and -8.4 eV, which may be due to O-O interaction. Larger O₂ exposures yield a valence band with structure at -3.2, -5.3, -7.2, and -9.5 eV.