Metal cluster source sample

The thennal evaporation source was the earliest used to produce metal clusters in the gas phase [H, 12 and 13], mostly for clusters of the alkalis and other low melting point materials. In this technique, a bulk sample is simply... 

There are several preparative methods for the production of bare metal clusters including the fast flow reactor (PER), the fast flow tube reactor (FTR), the SIDT (24), the GIB (23), and a supersonic cluster beam source (SCBS) (198). Essentially, all of these methods are similar. The first process is to vaporize the metal sample producing atoms, clusters, and ions. Laser vaporization is generally favored although FAB or FIB may be used. The sample is located in a chamber or a tube and so vaporization generally takes place in a confined environment. An inert gas such as helium may be present in the vaporization source or may be pulsed in after the ionization process. 

The other chief comptment of a mass spectrometer, the ion source, determines the types of ions that can be examined when starting fi om a specific sample (Gross and Caprioli, 2007). Laser-based methods, variably dubbed ablation, ionization, and desorption/ionization supposedly depending on the involved laser power, have played an important role fi om initial studies of bare metal ions to the widespread sought-after production of cluster ions for these latter species, the development of the so-called cluster sources was key to progress (Duncan, 2012). In more recent years, electrospray ionization (ESI) has played a central role due to its capacity to transfer/produce ions from solutions under mild conditions. Besides yielding new types of ions for chemical probing, ESI became a method of choice for the identification of solution species (speciation) and for direct observation of reaction... 

Palladium clusters deposited on amorphous carbon have been studied by XPS and UPS [28] and both techniques show broadening of the d-band peak as cluster size increases. The d-threshold shifts towards Ep as cluster size increases. In UPS studies the d-emission of the single atom has its peak at 3.0 eV below Ep, whereas the d-threshold is 2eV below Ep. Palladium clusters evaporated onto Si02 have been studied by UPS [38]. At large coverages of the Pd metal evaporated (> 10 atoms/cm ), a high emission intensity at Ep excited with photons of 21.2 eV (He(I)) or 40.8 eV (He(II)) as excitation source, is observed. This feature is characteristic in the spectra from bulk Pd samples. At the lowest metal coverage (3 x 10 atoms/cm ),... 

Sample. This source places no restrictions on target material. Clusters of metals, produced. For example, polyethylene and alumina have been studied as well as refractory metals like tungsten and niobium. Molecular solids, liquids, and solutions could also be used. However the complexity of the vaporization process and plasma chemistry makes for even more complex mixtures in the gas phase. To date the transition metals(1-3) and early members of group 13 (IIIA) and 14 (IVA)( 11-16) have been the most actively studied. 

The laser vaporization approach allows the use of even the most refractory target materials. The source configuration used in Fig. 1 involves a target rod that is rotated and translated in a continuous screw motion to expose fresh metal to the laser beam. This has been found necessary to provide acceptable pulse-to-pulse reproducibility. Target rods of refractory metals, semiconductors, carbon, polyethylene, alumina, and alloys have all been vaporized successfully to make clusters in many laboratories. For some materials a disk target is preferred due to the ease in sample preparation. Molecular solids, liquids, and solutions could also be used, though care must be taken to consider the additional complex plasma chemistry one is likely to encounter. 

Let us consider silver particles prepared on a model system for our experimental studies. The clusters were prepared inside the spectrometer by evaporation from thermally heated tungsten sources using high-purity wires. The substrate used was carbon prepared by prior evaporation onto mica with the technique and equipment described by Hamilton et al. (4 ). Upon introduction into the spectrometer, the carbon film was ion-etched with Xe, to remove traces of or S. These samples remained clean for a few days before use as substrates for the evaporated metal. 

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