CHEMICAL-REACTIONS OF SMALL GAS-PHASE METAL-CLUSTERS

The study of gas phase chemical reactions of neutral metal clusters with simple molecules has become feasable due to the development of the pulsed fast flow reactor and the low pressure continuous flow tube reactor. In both devices a laser vaporization cluster source is coupled to the flow tube reactor. An inert carrier gas enables both cluster formation and a continuously variable concentration of the reagent. Upon exciting the reactor the product composition is frozen in an adiabatic expansion. Clusters and reaction products are formed into a molecular beam and detected by pulsed laser ionization and time-of-flight mass spectrometry. This contribution focuses mainly on the performance of the continuous flow tube reactor. This device offers the unique advantage that a quantitative analysis of the reactions becomes principally possible, because the clusters can be exposed to controlled concentrations of reagent for a determinable time interval at a controlled temperature. It is possible to distinguish between kinetically and thermodynamically controlled reactions. One of the major problems in the data analysis is to relate the detected distribution of cluster ions to the original distribution of the neutrals. Fragmentation of the clusters, differences in ionization efficiencies, and desorption of adsorbates can distort the initial distribution. A rate equation analysis allows us to relate the observed ion intensities to the abundance distribution of the neutral clusters. The reaction of iron clusters with D2O is discussed as an application.