Oxygen adsorption on molybdenum studied by low-energy secondary-ion mass spectrometry and electron-induced desorption

The adsorption of oxygen on molybdenum in the monolayer region was studied using a combination of secondary-ion mass spectrometry (SIMS) at low primary energies and electron impact desorption. By examining the ion yields during adsorption and desorption at various temperatures, it was possible to identify two strongly bound states with activation energies for desorption of 120 and 107 kcal/mole. At room temperature, these $\beta$ states are populated first and afterwards there are at least two $\alpha$ states which are simultaneously filled. The desorption energies for the $\alpha$ states were estimated to be about 37 kcal/mole. One $\alpha$ state gives rise to electron-induced desorption of ${\mathrm{O}}_e^{+}$, the other does not. After only partial filling of the $\alpha$ states, a transformation of the state giving ${\mathrm{O}}_e^{+}$ could be observed at quite low temperatures (> 550°K). The rate of transformation was greater, the lower the degree of occupation of the $\alpha$ states. The ${\beta }_1$, ${\beta }_2$, and $\alpha$ states have quite different influences on the SIMS ion yields. The formation of the oxide produces characteristic changes in ion yields. Secondary-ion energies were also measured. The value of low-energy SIMS in characterizing different states of adsorption is demonstrated.