Interaction of O$_2$ with Silver Cluster Cations

The interaction of oxygen molecules with silver cluster cations, Ag$_n^+$ for n=8-11, is investigated using the first- principles calculations based on the density functional theory with generalized gradient correction. The binding energy of single O$_2$, either molecularly or dissociatively adsorbed on Ag$_n^+$, exhibits an even-odd alternation as a function of n, where O$_2$ binds more strongly for even n. When two oxygen molecules are adsorbed, the dissociative adsorption of the first molecule favors the molecular adsorption of the second one. Analyses of the local density of states (LDOS) and the spatial distribution of the highest occupied molecular orbital (HOMO) are presented to explain our observations.