Chemisorption of an oxygen molecule on a silver surface is studied theoretically with the use of the dipped adcluster model (DAM) for AgxO2 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 (O2-2) 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.
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