Exoelectron emission during the oxidation of Na films

Oxidation of Na films is accompanied by a low yield of electron emission which is, however, confined to the later stages of reaction in which transformation of peroxide (Na2O2) into Superoxide (NaO2) species at the surface takes place. By probing the electronic properties of the outermost layer by means of metastable deexcitation spectroscopy (MDS) and by recording the energy distribution of the emitted exoelectrons the mechanism of this process was found to be analogous to that established previously for the oxidation of Cs films and as proposed earlier theoretically. It involves decay of a hole state derived from the affinity level of the impinging O2 molecule in front of the surface via an Auger transition which, on the other hand, may efficiently be quenched by resonance ionisation from metallic electrons near the Fermi level. Experiments with Na submonolayers adsorbed on a Ru(0001) substrate reveal that for coverages < 0.6 ML the latter effect dominates so strongly that exoelectron emission is no longer observed.