Cluster beam techniques

It had been expected, before the first macroscopic production and extraction of La Cs2 (Chai et al., 1991), that metallofullerenes based on the Cgo cage would be the most abundant metallofullerenes that were prepared in macroscopic amoimts, as was the case in empty fullerenes. This is simply because that Ceo is the most abundant fullerene which can be easily produced by either the arc-discharge or the laser furnace method (cf. Section 2.1). In fact, an earlier gas phase experiment on the production of carbon clusters containing La via the laser-vaporization cluster-beam technique (Heath et al., 1985) indicated that La Cgo is a prominent "magic number" species among various La C (44 < n < 80) clusters (Figure 1). 

As beautiful as this suggestion might be, significantly more must be done to confirm it than to observe that the combination of sixty carbon atoms can be produced in greater abundance than other numbers. Unfortunately, the laser vaporization supersonic cluster beam technique does not produce enough material to perform direct structural techniques like x-ray crystallography or even indirect (but often decisive) experiments such as infrared or Raman spectroscopy, or nuclear magnetic resonance. 

Takeuchi, M., Yamashita, H., Matsuoka, M., Anpo, M., Hirao, T., Itoh, N., and Iwamoto, N. Photocatalytic decomposition of NO on titanium oxide thin film photocatalysts prepared by an ionized cluster beam technique. Catal. Lett. 2000,... 

Feng SW, Nainaparampil JJ, Tabet MF, Urban FK (1994) Gold and zinc thin films deposited by the ionized cluster beam technique. Thin Solid Films 253 402-406... 

The talk will briefly review some of these developments ranging from high temperature equilibrium plasmas to cool plasmas, PECVD, ion implantation, ion beam mixing and ion assisted etching and deposition. Brief consideration will also be given to sputtering and ionised cluster beam deposition techniques in inorganic synthesis. 

Effusive beam technique, 157-158 Electron bombardment flow radiolysis, 238 Electrospray ionization and ionic clusters, 168 Enantiomers, separation techniques, 154-155 Enantioselectivity of enzymes, 148 Enthalpy-entropy compensation plots, 261 Enthalpy of activation, and quantum tunneling, 67, 70-71... 

Mass spectrometric investigation of gas phase clusters has come to play a significant role in cluster research because it is presently the only technique that allows size-dependent analysis of cluster beams. We anticipate that, in the near future, CMS studies utilizing tandem mass spectrometric techniques will be invaluable in confirming the importance of metastable decompositions and in determining the factors responsible for the enhanced intensities of various ions in the CMS. Such an instrument is currently under construction in our own labs, and we should soon have the ability to perform collision-induced dissociation studies on large vdW clusters. 

CoO-coated Co cluster and oxide-coated Fe cluster assemblies were prepared by a plasma-gas-aggregation cluster-beam-deposition technique [37-39]. For preparation of CoO-coated Co cluster assembly, oxygen gas was introduced through a nozzle near the skimmer into the deposition chamber. The Co clusters with CoO shells were formed before deposition onto the substrate [37], Figure 8 shows a TEM image of the clusters produced at oxygen gas flow rate R(02) = 1 seem. Clusters are almost monodispersed, with the mean diameter of about 13 nm. Electron diffraction pattern indicated the coexistence of Co and CoO phases. The cluster assemblies were formed on a polyimide film with a thickness of about 100 nm. 

In Volume 21, Part A, the preparation of a-Si H by rf and dc glow discharges, sputtering, ion-cluster beam, CVD, and homo-CVD techniques is discussed along with the characteristics of the silane plasma and the resultant atomic and electronic structure and characteristics. 

The preliminary studies described above indicate the power of combining molecular-beam techniques for synthesizing metal clusters of known size and composition and techniques for studying individual supported clusters. It is to be expected that this fusion of experimental methods will lead to increased understanding of the complex world of supported metal catalysts. 

To illustrate the prominent features in the reactivity of supported nanosized metal clusters, we focus on two reactions that have recently been studied in detail mainly by molecular beam techniques the CO oxidation and the NO reduction by CO on supported Pd clusters. First we discuss the adsorption of the reactants and then the catalytic reaction itself. 

As for CO [171,172,176], the adsorption of NO as been studied on Pd supported on MgO(lOO) using molecular beam techniques [184]. The scattering of NO on the clean MgO surface shows that 56% of the NO beam becomes physisorbed while 44% of the beam is quasi-elastically reflected [184]. When Pd clusters are present on the substrate, NO becomes becomes chemisorbed on them and, as in the case of CO (see former paragraph), part of the NO physisorbed on the MgO is caught by the Pd clusters and becomes chemisorbed [184]. The lifetime of NO molecules chemisorbed on the Pd clusters has been measured at various temperatures in modulating the beam by a square wave. At low coverage, the lifetime as a function of temperature... 

The NO reduction by CO is a very important reaction taking place in the car exhaust catalytic converters. It has received considerable attention on Rh single crystals [210] and much less on Pd [211,212]. On supported model catalysts many fewer experiments have been performed than for CO oxidation, and only in recent years [212-216]. From the work from Goodman s group it appears that the reactivity of Pd single crystals decreases when the surfaces become more open, and on supported clusters it decreases with cluster size [212]. We have studied this reaction by molecular beam techniques [209]. The steady-state reaction rate as a function of... 

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