GAS-PHASE CHEMISTRY OF MOLYBDENUM CLUSTERS

The kinetics of reactions of molybdenum clusters, Mo-n, n = 1-25, in t he pressure range 0.4-4 Torr, and temperature range 270-380 K, have be en investigated using a large-bore, He-buffered, fast-flow reactor equ ipped with a laser-vaporization source for the production of clusters. The reactor is designed to make kinetic measurements on neutral metal clusters in the gas phase under well-defined pressures and temperatur es. We discuss a new version of the instrument in which LIF techniques , used previously to monitor atoms and dimers, are replaced by laser i onization, time-of-flight mass spectrometry (TOFMS) in order to monito r larger clusters. The new version of the reactor has been tested agai nst known reactions of Ti atoms. Examples of the reactor's performance are taken from studies performed on Mo-n cluster reactivity. In parti cular we summarize some results on the dissociative chemisorption of m olecular nitrogen, where large cluster-size effects are found. In some cases a negative-temperature dependence of the kinetics indicates the involvement of a precursor bound state and leads to conclusions conce rning the shape of the potential-energy surface and how subtle changes associated with the cluster's geometric structure might profoundly al ter reaction rates.