Abstract: Negative ion formation in O2 adsorbed in Pt(111) is investigated using a layer-KKR calculation in which the unoccupied electronic structure of the substrate is fully incorporated. A significant variation of the lifetime the (4)Sigma_u(-) shape resonance is observed as the adsorption geometry of the molecule is altered. Maps of the resonance lifetime reveal sites of partial activation and quenching of the resonant state. The physical origin of the observed behavior is described by a tunneling theory of resonance formation at surfaces.