Molecular clusters sources

To date, the IR-CRLAS studies have concentrated on water clusters (both FI2O and D2O), and methanol clusters. Most importantly, these studies have shown that it is in fact possible to carry out CRLAS in the IR. In one study, water cluster concentrations in the molecular beam source under a variety of expansion conditions were characterized [34]- hr a second study OD stretching bands in (020) clusters were measured [35]. These bands occur between 2300... 

The fonnation of clusters in the gas phase involves condensation of the vapour of the constituents, with the exception of the electrospray source [6], where ion-solvent clusters are produced directly from a liquid solution. For rare gas or molecular clusters, supersonic beams are used to initiate cluster fonnation. For nonvolatile materials, the vapours can be produced in one of several ways including laser vaporization, thennal evaporation and sputtering. 

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. 

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... 

Figure 3.9. Transient C02 formation rates on Pd30 (a) and Pd8 (b) mass-selected clusters deposited on a MgO(lOO) film at different reaction temperatures [74]. In these experiments CO was dosed from the gas background while NO was dosed via a pulsed nozzle molecular beam source. The turnover frequencies (TOFs) calculated from the experiments displayed in (a) and (b) are displayed in the last panel (c). C02 formation starts at lower temperatures but reaches lower maximum rates on the larger cluster. (Figure provided by Professor Heiz and reproduced with permission from Elsevier, Copyright 2005).

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... 

Copper selenide is also used in solar cells [200]. TOPO-capped CuSe nanoparticles were synthesized from Cu(Se2CNEt2)2 using a single-source method [201]. Copper selenide nanoparticles have also been prepared and XPS spectra, valence bands and Auger transitions reported for the molecular clusters Cui46Se73(PPh(3))(202) and Cu2Se [203]. Photoemission features were reported for both species. 

Rohlfing EA (1988) Optical-emission studies of atomic, molecular, and particulate carbon produced from laser vaporization cluster source. J Chem Phys 89(10) 6103-6112... 

The first chamber hosts the cluster source, evacuated by two turbo-molecular pumps (3001/s). They are horizontally mounted facing each other, in order to assure the correct operating conditions for the pulsed supersonic... 

While the use of direct absorption methods has grown, indirect action spectroscopic methods continue to be widely and successfully used in the study of neutral molecular clusters. As mentioned earlier, there are two commonly used detection methods, mass spectrometers and bolometers. Because of the variety of mass-spectroscopic methods, there is an equally wide range of techniques used in neutral cluster spectroscopy. One of the oldest among these involves electron-impact mass spectrometry of a cw neutral beam combined with vibrational predissociation spectroscopy using a tunable cw or pulsed laser. The advent of continuously tunable infrared sources (such as color center lasers and LiNbOa optical parametric oscillators) allowed for detailed studies of size and composition variation in neutral clusters. However, fragmentation of the clusters within the ionizer of the mass spectrometer, severely limited the identification of particular clusters with specific masses. Isotopic methods were able to mitigate some of the limitations, but only in a few cases. 

Recent progress in this field has been made by using high spectral resolution from light sources that rely on insertion devices, such as undulators. Small spectral shifts are found in the soft X-ray regime of molecular clusters that include vibrational resolution of excited valence states. 

Cluster analysis is therefore a tool of discovery. It may reveal associations and structure in data which, though not previously evident, nevertheless are sensible and useful once found. The results of cluster analysis may contribute to the definition of a formal classification scheme, such as molecular marker source confirmation, or indicate rules for assigning new cases (samples) to classes (groups) for identification and diagnostic purposes. 

FIGURE 8.6 Facetted molecular clusters of a- (a) and (I-forms (b) and energy minimisation (c) of relaxed facetted clusters of l-GA. Source Adapted from Hammond et al. [70]. Reproduced from Elsevier. 

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