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Laser vaporization of a substrate within the throat of a pulsed nozzle is used to generate a supersonic beam of carbon clusters. The neutral cluster beam is probed downstream by UV laser photoionization with time-of-flight mass analysis of the resulting photoions. Using graphite as the substrate, carbon clusters Cn for n=1–190 have been produced having a distinctly bimodal cluster size distribution: (i) Both even and odd clusters for Cn, 1≤n≤30; and (ii) only even clusters C2n, 20≤n≤90. The nature of the bimodal distribution, and the intensity alterations in the observed C+n signals are interpreted on the basis of cluster formation and stability arguments. Ionizing laser power dependences taken at several different photon energies are used to roughly bracket the carbon cluster ionization potentials, and, at high laser intensity, to observe the onset of multiphoton fragmentation. By treating the graphite rod with KOH, a greatly altered carbon cluster distribution with mixed carbon/potassium clusters of formula K2C2n is produced.

We have examined the reactivity and saturation of small gold clusters (cations, neutrals and anions) towards several molecules and find that specific small gold clusters exhibit a pronounced variation in their reactivity towards hydrogen, methane and oxygen. The reactivity not only depends strongly on cluster size but also on the cluster charge state. For example, small (n < 15) gold cations react readily with Dz, but no evidence of reaction is observed for the anions under our experimental conditions. Similar behavior is seen for methane. With oxygen only even atom (odd electron) anions are reactive, and Auto is the only cation which exhibits evidence of reaction. The global features (small cluster cations reactive towards H2, CH4, but large ones not reactive, odd electron anions reactive towards O2) are qualitatively explained by appealing to a simple frontier orbital picture. The uptake of deuterium and methane on gold clusters also exhibits a pronounced size dependence with D/Au varying from a high of 3 for the dimer to zero for clusters containing more than 15 Au atoms. Comparison of the methane and deuterium saturation behavior leads us to suggest that methane is dissociated and bound as CH 3 and H.

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New experiments in a fast-flow reactor have uncovered a strong correlation between the reactivity of free iron clusters and cluster ionization thresholds: Clusters with low thresholds efficiently add molecular hydrogen, and the relative rates of this reaction closely follow variations in clusterelectron binding energy. This correspondence can be understood in terms of a requirement for metal-to-hydrogen charge transfer in the activation of the H 2 bond.

Using the pulsed cluster beam flow reactor technique, we have measured the reactivity of aluminum clusters toward several molecules under thermal conditions. For each different molecule we observe that the reactivity exhibits a unique dependence on the number of aluminum atoms in the cluster. The overall reactivity of aluminum clusters toward different molecules is ordered roughly as 02 > CH3OH > CO > D20 > D2 > CH4 with CH4 showing no reaction under these experimental conditions. In addition, we are able to place upper and lower bounds on the ionization thresholds for the smaller aluminum clusters.