Composition dependent adsorption of multiple CO molecules on binary silver-gold clusters AgnAum+ (n plus m=5): theory and experiment

The binding energies of multiple CO molecules to five-atom silver-gold cluster cations have been obtained employing temperature dependent gas phase ion trap measurements and ab initio calculations. The CO binding energies to AgnAum+ (n + m = 5) decrease with increasing number of silver atoms. Most strikingly, after the adsorption of the fourth CO to Au-5(+) and of the third CO to Ag-5(+), respectively, a pronounced decrease in the binding energies of further CO molecules was observed. This is related to a CO-induced structural transformation yielding more compact metal cluster geometries. First principles calculations revealed that the exact structure of the carbonyl complexes with multiple CO and the nature of the CO-induced structural transformation strongly depend on the composition of the metal cluster as well as on the number of adsorbed CO molecules.