Theoretical study on molecular and dissociative chemisorptions of an O₂ molecule on an Ag surface: dipped adcluster model combined with symmetry-adapted cluster-configuration interaction method

Chemisorption of an oxygen molecule on a silver surface is studied theoretically with the use of the dipped adcluster model (DAM) for Ag_xO₂ with x=2 and 4. Electron correlation in low-lying surface states and electron transfer from bulk metal are described by combining DAM with the symmetry-adapted cluster (SAC) CI method. For chemisorption, electron transfer from the bulk metal to the adcluster and the electrostatic image force are important. They cannot be treated by the conventional cluster model, but are dealt with by the DAM. Three low-lying states, ²A₁, ²A₂, and ²B₁, of the adclusters Ag₂O^-₂ and Ag₄O^-₂ are involved in the chemisorption. In the molecular adsorption state, the ²A₁ state is assigned to peroxide (O^2-₂) species and the ²A₂ and ²B₁ states to superoxide (O^-₂) species. The OO stretching frequencies are in agreement with the experimental values. In the potential energy curves of Ag₄O^-₂, another potential minimum corresponding to the dissociative adsorption state is obtained for the ²A₁ state. The energetics of the molecular and dissociative adsorptions show reasonable agreement with experiments.