Size dependent reaction kinetics of small gold clusters with carbon monoxide:: Influence of internal degrees of freedom and carbonyl complex stability

The reactions of free gold clusters Au-, Au-2(-), and Au-3(-) with carbon monoxide are studied in an rf-octopole ion trap experiment at cryogenic temperatures. While Au- is unreactive toward CO over the whole temperature range investigated, the two and three atom cluster anions show a maximum adsorption of two CO molecules at temperatures below 250 K. From time resolved trapping experiments the strongly cluster size dependent reaction kinetics are obtained and a distinct reaction mechanism is deduced. The size dependence of the measured rate coefficients reveals the preferred formation and particular stability of the carbonyl complex Au-3(CO)(2)(-). Through RRK analysis of the absolute termolecular rate coefficients we are able to estimate the binding energy of CO to Au-2 and Au-3.