Electron-stimulated desorption of both positive and negative ions from oxygen-covered Mo (100) surfaces was investigated. Two adsorption states, and , were identified. For the high-ion-yield state, the O- to O+ yield ratio varied with both temperature and oxygen coverage, indicating changes in the nature of this state. For the low-ion-yield state, the 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. © 1979 The American Physical Society.