Mass-selected, monodispersed

The next section will describe the experimental techniques we use to synthesize and study clusters and present typical data. The following section will discuss the cluster size sensitive behavior observed in kinetic studies of H-H and C-H bond activation reactions as well as the size sensitive behavior of hydrogen uptake, and discusses the potential implications of these experiments in catalysis and chemisorption. The last section gives the highlights of recent studies of the electronic properties of mass selected, monodispersed, platinum clusters containing up to 6 Pt atoms supported on Si02. 

As discussed earlier, it is now possible to make and study deposits of monosized, highly dispersed, transition metal clusters.(S) In this section we summarize results from the first measurements of the valence and core level photoemission spectra of mass selected, monodispersed platinum clusters. The samples are prepared by depositing single size clusters either on amorphous carbon or upon the natural silica layer of a silicon wafer. We allow the deposition to proceed until about 10 per cent of the surface in a 0.25 cm2 area is covered. For samples consisting of the platinum atom through the six atom duster, we have measured the evolution of the individual valence band electronic structure and the Pt 4f... 

A further less commonly employed but very classical method for mass-selection is the Wien-filter which combines an electrostatic and a magnetic field for mass-selective cluster deflection. This device is applied in particular with the purpose to obtain large cluster currents for cluster-surface interaction and deposition experiments at moderate resolution [75, 76]. Another approach for size-selection in combination with a sputter source ]47] or a laser vaporization source [61,77,78] has been chosen recently. In both cases, a magnetic dipole field has achieved the size-selection. Subsequent deceleration of the size-selected clusters to less than 1 eV per atom enables soft-landing under UHV conditions [47]. Monodispersed chromium cluster beam densities range from 0.1 to 5nA mm, depending on the cluster size ]47]. 

The rates of multiphase reactions are often controlled by mass tran.sfer across the interface. An enlargement of the interfacial surface area can then speed up reactions and also affect selectivity. Formation of micelles (these are aggregates of surfactants, typically 400-800 nm in size, which can solubilize large quantities of hydrophobic substance) can lead to an enormous increase of the interfacial area, even at low concentrations. A qualitatively similar effect can be reached if microemulsions or hydrotropes are created. Microemulsions are colloidal dispersions that consist of monodisperse droplets of water-in-oil or oil-in-water, which are thermodynamically stable. Typically, droplets are 10 to 100 pm in diameter. Hydrotropes are substances like toluene/xylene/cumene sulphonic acids or their Na/K salts, glycol.s, urea, etc. These. substances are highly soluble in water and enormously increase the solubility of sparingly. soluble solutes. 

The Pd clusters have been produced by a recently developed high-frequency laser evaporation source, ionized, then guided by ion optics through differentially pumped vacuum chambers and size-selected by a quadrupole mass spectrometer [16-18]. The monodispersed clusters have been deposited with low kinetic energy (0.1-2 eV) onto a MgO thin film surface. The clusters-assembled materials obtained in this way exhibit peculiar activity and selectivity in the polymerization of acetylene to form benzene and aliphatic hydrocarbons [30]. 

If we assume that the molecules in each slice, /, following separation by SEC, are monodisperse, the mass moments of each sample peak selected are calculated from the conventional definitions [3,5] by... 

Combined SEC-MALDI. Mark-Houwinkr-Sakurada (MHS) Parameters As mentioned earlier, combination of the quantitatively reliable LC techniques with the identification power of MS was used to overcome the limitations inherent in the use of either LC or MS alone. In addition to ESI-MS/SEC, combined applications already mentioned, several approaches to SEC/ MALDI time-of-flight (TOF) MS coupling have also been reported [78-83]. In the case of polydisperse polymers, determination of molecular mass by SEC/MALDI-TOF involves the fractionation of the samples through an analytical SEC. Selected fractions are then analyzed by MALDI-TOF and the mass spectra of these nearly monodisperse samples allow the determination of the MWD moments. Montaudo et al. [81] have tested the reliability of the method for different polydisperse samples, such as poly(methylmethacrylate) (PMMA), poly(dimethylsiloxane) (PDMS), and copolyesters. 

The materials used in this study were atactic polystyrenes including a wide range of monodisperse linear PS (PDI = MJM < 1.15), a selection of monodisperse 3 and 4-arm star PS [5], and several linear polydisperse PS. Molar mass measurements performed by GPC are shown in Table 1. 

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