The O sub 2 --Ag(111) Interaction Studied by 100-3000 eV Glancing Incidence O sub 2 exp + Scattering

The technique of specular scattering of energetic beams from carefully flattened surfaces is applied to investigate the dynamics of the O sub 2 --Ag(111) adsorption/reaction system. The experiments were performed using 100--3000 eV beams of O sub 2 exp + incident at 85 deg to the surface normal. The scattering products O, O sub 2 , O exp -- and O sub 2 exp -- were observed using time-of-flight techniques. Part of the incidenct O sub 2 exp + beam dissociates via charge transfer to dissociative or predissociative states of O sub 2 . Charge transfer to the ground electronic state of O sub 2 , if accompanied by the transfer of a second electron, results in O sub 2 exp -- formation. Above a threshold energy of 180 eV, the probability of formation of vibrationally cold O sub 2 exp -- is high ( > 5%), indicating a large charge transfer probability at the surface and efficient access of the O sub 2 exp -- region of the adsorption/reaction potential energy surface (PES). At an incident beam energy of 2000 eV a threshold for O exp -- production is observed. Classical trajectory calculations performed for the scattering of O sub 2 from Ag(111) indicate that below 2000 eV no dissociation of the molecular occurs and that even about a fraction of a third of the molecules is vibrationally cold. Above 2000 eV collision induced dissociation of the O sub 2 molecule is observed in the calculations that bears a striking resemblance to the experimentally observed O exp -- yield. Therefore, it is proposed that O sub 2 exp -- dissociates as a result of the impulsive collision with the surface, resulting in the observed O exp -- production. It is concluded from the results that O sub 2 exp -- is a precursor to dissociative adsorption and that the O exp -- region of the PES is not readily accessible in the experiment, despite the presence of sufficient translational energy to overcome all activation batteries along the reaction coordinate. 64 ref.--AA