Cluster sources

Figure Cl. 1.1. Schematic of a typical laser vaporization supersonic metal cluster source using a pulsed laser and a pulsed helium carrier gas.

The size distribution of the clusters produced in the cluster source is quite smooth, containing no information about the clusters except their composition. To obtain information about, for example, the relative stability of clusters, it is often useful to heat the clusters. Hot clusters will evaporate atoms and molecules, preferably until a more stable cluster composition is reached that resists further evaporation. This causes an increase in abundance of the particularly stable species (i.e., enhancing the corresponding peak in the mass spectrum, then commonly termed fragmentation spectrum ). Using sufficiently high laser fluences (=50 /iJ/mm ), the clusters can be heated and ionized simultaneously with one laser pulse. 

The pulsed molecular beam cluster source has produced clusters of virtually every material—we have made clusters of even the most refractory transition metals, of group IIIB and IVB elements, and numerous oxides, carbides, and intermetallic alloys of these elements. 

Figure 1. Schematic illustration of the laser-vaporization supersonic cluster source. Just before the peak of an intense He pulse from the nozzle (at left), a weakly focused laser pulse strikes from the rotating metal rod. The hot metal vapor sputtered from the surface is swept down the condensation channel in dense He, where cluster formation occurs through nucleation. The gas pulse expands into vacuum, with a skinned portion to serve as a collimated cluster bean. The deflection magnet is used to measure magnetic properties, while the final chaiber at right is for measurement of the cluster distribution by laser photoionization time-of-flight mass spectroscopy.

The final probe of molecular clusters is that of selected chemical reactions. The use of probe reactions to study supported cluster catalysts is well established, and we are attempting the development of similar probes of unsupported clusters. The first steps in this direction are the design of a pulsed chemical reactor to go with the pulsed cluster source and the development of criteria for reactions. It is important to recall that at present... 

The cluster reactor is attached to the pulsed cluster source s condensation channel, as shown in Figure 6. (16) To it is attached a high-pressure nozzle from which a helium/hydrocarbon mixture is pulsed into the reactor at a time selected with respect to the production and arrival of the clusters. The effect of turbulent mixing with the reactant pulse perturbs the beam, but clusters and reaction products which survive the travel from the source to the photoionization regime ( 600y sec) and the photoionization process are easily detected. 

Figure 6. Scale-drawn schematic of the cluster reactor in relation to the pulsed cluster source. The letters A-F indicate the various stages of cluster preparation or synthesis, cooling, mixing and reacting, and finally flowing into vacuum toward detection.

This overview is organized into several major sections. The first is a description of the cluster source, reactor, and the general mechanisms used to describe the reaction kinetics that will be studied. The next two sections describe the relatively simple reactions of hydrogen, nitrogen, methane, carbon monoxide, and oxygen reactions with a variety of metal clusters, followed by the more complicated dehydrogenation reactions of hydrocarbons with platinum clusters. The last section develops a model to rationalize the observed chemical behavior and describes several predictions that can be made from the model. 

Atomic and molecular clusters have been studied for more than fifty years, but the last two decades have seen an increasing interest in new experimental methods for cluster production and analysis. The development and improvement of cluster sources lie at the focal point of the technological advances achieved in the study of gas phase clusters. For what concern the molecules of biological interest, the production and analysis of these molecules both isolated or complexed is made... 

There are many more types of cluster ion sources than there are neutral cluster sources, in part because of the wide variety of ion-forming techniques available and in part because cluster ion sources are often hybrids of established neutral cluster sources and ion-forming environments. Some non-hybrid ion sources which can generate cluster ions include ion sputter, flow tube. Penning... 

As for neutral cluster sources, an ever growing array of techniques are being implemented to generate ionic clusters and measure their properties. Mass spectrometry is useful for quantitative determination of atoms or molecules and... 

However, it is rare when all the atoms in an aromatic species are metals. One such system was synthesized in 2001 by A. I. Boldyrev andL.-S. Wang and their colleagues. Using a laser vaporization supersonic cluster source and a Cu/Al... 

Formation of metal clusters by gas aggregation, in which metal atoms are evaporated or sputtered into a cooled inert gas flow at relatively high pressure, has been well established in last decade. By repeated collisions with the carrier gas, the supersaturated metal vapor nucleates and forms clusters. The mechanism of cluster formation can be explained with homogeneous and heterogeneous nucleation theories. The gas aggregation methods have been applied extensively to produce small clusters of metals such as zinc, copper, silver etc. [23-26]. In some cases this method was used in combination with a mass filter such as a quadruple or a time-of-flight spectrometer [27, 28], The metal vapor for cluster source can be produced by either thermal evaporation [23-28] or sputter discharge [22, 29]. 

Figure 13 shows the sputtering power dependence of cluster size. It can be seen that FePt, CoPt cluster size has a nearly linear relationship with the sputtering power of the cluster source. It is noted that the cluster size follows a Gaussian distribution rather than a lognormal one as observed in Fe clusters [31]. This may be related to the cluster formation mechanism involved in the present method, which is different from the mechanisms for lognormal distribution. A similar Gaussian distribution was also reported in CoAl clusters [40]. 

---