Charging effects on bonding and catalyzed oxidation of CO on [Au.sub.8] clusters on MgO

Gold octamers ([Au.sub.8]) bound to oxygen-vacancy F-center defects on Mg(001) are the smallest clusters to catalyze the low-temperature oxidation of CO to C[O.sub.2], whereas clusters deposited on dose-to-perfect magnesia surfaces remain chemically inert. Charging of the supported dusters plays a key role in promoting their chemical activity. Infrared measurements of the stretch vibration of CO adsorbed on mass-selected gold octamers soft-landed on MgO(001) with coadsorbed [O.sub.2] show a red shift on an F-center-rich surface with respect to the perfect surface. The experiments agree with quantum ab initio calculations that predict that a red shift of the C-O vibration should arise via electron back-donation to the CO antibonding orbital.