Observation of positive and negative oxygen ions during electron bombardment of oxygen-covered Mo (100) surfaces

Electron-stimulated desorption of both positive and negative ions from oxygen-covered Mo (100) surfaces was investigated. Two adsorption states, $\alpha$ and $\beta$, were identified. For the high-ion-yield $\alpha$ state, the ${\mathrm{O}}^{-}$ to ${\mathrm{O}}^{+}$ yield ratio varied with both temperature and oxygen coverage, indicating changes in the nature of this state. For the low-ion-yield $\beta$ state, the ${\mathrm{O}}^{+}$ -ion yield showed reversible thermal enhancement, possibly caused by the migration of oxygen atoms in and out of the Mo surface. A velocity-dependent isotope effect was also observed for both positive and negative oxygen ions. The phenomenon is discussed in the framework of Franck-Condon transitions.