Eur. Phys. J. D 24, 327–330 (2003) DOI: 10.1140/epjd/e2003-00185-6 T HE EUROPEAN P HYSICAL JOURNAL D Size dependent reaction kinetics of small gold clusters with carbon monoxide: Influence of internal degrees of freedom and carbonyl complex stability J. Hagen 1 , L.D. Socaciu 1 , U. Heiz 2 , T.M. Bernhardt 1, a , and L. W¨ oste 1 1 Institut f¨ ur Experimentalphysik, Freie Universit¨at Berlin, Arnimallee 14, 14195 Berlin, Germany 2 Abteilung f¨ ur Oberfl¨achenchemie und Katalyse, Universit¨at Ulm, 89069 Ulm, Germany Received 10 September 2002 Published online 3 July 2003 – c  EDP Sciences, Societ`a Italiana di Fisica, Springer-Verlag 2003 Abstract. 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 Au3(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 . PACS. 36.40.Jn Reactivity of clusters – 82.30.Fi Ion-molecule, ion-ion, and charge-transfer reactions – 82.80.Ms Mass spectrometry (including SIMS, multiphoton ionization and resonance ionization mass spectrometry, MALDI) 1 Introduction The remarkable size and charge state dependence of the reactivity of free gold clusters toward small molecules was first recognized more than a decade ago [1]. The close rela- tion of gold cluster electron affinity and reactivity toward molecular oxygen, e.g., has been demonstrated by several groups [1–4]. The reaction of gold cluster ions with carbon monoxide has also been reported and an enhanced stabil- ity of several particular carbonyls was observed [5,6]. CO binding to free clusters with less than four atoms, however, is known to be rather weak [3,7,8] and Au - 1-3 carbonyls could even not be detected in some experiments [6]. We re- cently reported on the low temperature synthesis and cat- alytic activity of free carbonyls of the negatively charged gold dimer and trimer in a radio frequency(rf)-ion trap experiment [4,9]. In the present contribution we now aim to reveal the formation mechanism of the mono- and di- carbonyl complexes of these small gold cluster anions. As gold nano-particles and clusters currently attract consid- erable interest for their low temperature catalyic activity in a variety of reactions including CO combustion [10], the reaction mechanism of small gold clusters with CO is of a e-mail: tbernhar@physik.fu-berlin.de particular importance to add to the understanding of the catalytic properties of these nano-scale gold materials. 2 Experiment To study the mechanism of Au - n cluster reactions, a vari- able temperature rf-octopole ion trap inserted into a tan- dem quadrupole mass spectrometer is employed. In com- parison to flow tube reactor experiments, the use of an rf-ion trap to investigate gas phase metal cluster reac- tions has the advantage that the experimental conditions, i.e., reaction temperature, reactant partial pressures, total pressure, and reaction time, can be precisely determined. The general experimental setup is described in detail else- where [11] and will only be outlined briefly here. The gold cluster anions are produced by an ion sputtering source. Mass selection is achieved via a first quadrupole mass fil- ter. The mass-selected cluster ion beam then enters the octopole ion trap, which is filled up to space charge limit with metal cluster ions (about 10 4 clusters per mm 3 ). The trap is prefilled with a helium partial pressure on the order of 1 Pa and the clusters are thermalized to the temper- ature of the background helium gas in the trap within few milliseconds. A closed cycle helium cryostat attached to the trap allows temperature adjustment in the range