Carbide cluster ions

Metal-carbide clusters are relevant to the fonnation of both endohedral fullerenes and carbon nanotubes [1351. There also exists a class of apparently stable metal-carbide cluster ions, = Ti, V, Cr, Zr and Hf), called... 

Part of a mass spectrum for the determination of Fe and Cr contamination in boron nitride contaminated with carbon measured by LIMS is shown in Figure 6.9. The analyte ions 53Cr+ and 54Fe+ due to different masses of isobaric atomic and cluster ions are clearly separated from boron and boron carbide cluster ions as demonstrated in Figure 6.9. Cluster ion formation has been studied by laser ionization mass spectrometry (LIMS) on a boron nitride target.10... 

Besides the carbon cluster ions, carbide cluster ions are formed in different plasmas with remarkably high ion intensities. The abundance distribution of these carbide cluster ions as a function of cluster size has been observed to be very similar using the different sohd-state mass spectrometric methods, with higher intensities for cluster ions with even numbered carbon atoms than for neighbouring... 

Carbide cluster ions (MC + - M = matrix element) have been measured by investigating them directly from the solid carbides (B4C,46 SiC) or by analyzing metal oxide/graphite mixtures (for M = rare earth element,3 Si,46 Th or U36). Figure 9.60 shows the distribution of silicon carbide cluster ions (SiC +) in laser ionization mass spectrometry by the direct analysis of compact SiC in comparison to the carbide cluster ion distribution of LaC + and SrC + in spark source mass spectrometry, by investigating a metal oxide/graphite mixture. 

Carbide cluster ions - M = matrix element) have been measured by investigating them... 

In the experiments discussed in earlier sections, the separation of the metal carbide cluster ions produced either directly in the source or in the carrier gas was exclusively performed on a mass criterion. Ion chromatography experiments performed by Bowers and co-workers introduce another selection criterion by allowing a mass-selected ion cloud to undergo collisions with a He buffer gas in a drift cell [28,68] given species will depend on its collision cross section... 

Several techniques have been used to investigate the reactivity of the metal carbide cluster ions formed in a laser vaporization source. The earliest investigations performed by Castleman s group relied on a preliminary mass selection of the desired cluster. The ion beam was then injected into a drift tube where the selected cluster encounters the reactant mixed with helium as a buffer gas. The FTICR (Fourier-transform ion cyclotron resonance) mass spectrometer studies reported by Byun, Freiser and co-workers basically rely on the same principle even though the total pressure of the reaction chamber is 10 torr, compared with 0.7 torr in Castleman s experiments. A new method of forming met-car ligand complexes was then reported by Castleman et al. this involved the direct interaction of the vaporized metal with mixtures of methane and selected reactant gases. ... 

Niobium and rhodium cluster anions have been prepared by laser vaporization and the reactions with benzene studied by FT-ICR/MS (58). The reactions of the anions and similar cations have been compared. With few exceptions the predominant reaction of the niobium cluster anions and cations was the total dehydrogenation of benzene to form the metal carbide cluster, [Nb C6]-. The Nb19 species, both anion and cation, reacted with benzene to form the coordinated species Nb 9C6I I6p as the predominant product ion. The Nb22 ions also formed some of the addition complex but the Nb2o Nb2i, and all the other higher clusters, formed the carbide ions, Nb C6. ... 

The influence of the laser and plasma parameters (such as wavelength, laser power density, pulse length, plasma temperature, electron and ion density and others) on the physical and chemical processes in a laser induced plasma with respect to the formation of polyatomic and cluster ions has been investigated for different materials (e.g. graphite, boron nitride, boron nitride/graphite mixture, boron carbide, tungsten oxide/graphite mixture and superconductors ). 

Methoxylation of the hexaruthenium carbide cluster [RugC(CO)i7] generates the stable anionic cluster [RugC(COOMe)(CO) g] (3a), the negative-ion EDESI mass spectrum of which is shown in Figure 5. ... 

The recently reported carbide clusters of ruthenium, Rue(CO)i7C and Ru6(CO)i4(arene)C (211), provided a good example of the power of mass spectrometry as an analytical tool. The appearance of the ions Rug(CO) C+ (n = 0-17) provided a clear differentiation between Rue(CO)i7C and another proposed formula Rug(CO)i8 (324). The first carbonyl oxide cluster 0s3(C0)i2 0s04 was characterized by its mass spectrum. The ions 0s404(C0),i+ (n = 0-12) occur in high abundance and the tetranu clear cluster does not rupture until all CO groups are lost (219,220). Other clusters which have been characterized by mass spectrometry include Os3(CO)io(OMe)2 (217), Ru3(CO)i2Xg (X = Cl, Br, I) (212),... 

It was later shown in two independent studies that the relative importance of NbmC met-cars and nanocrystals in the mass spectrum is critically dependent on the experimental conditions. It has been proved that the concentration of hydrocarbon in the carrier gas, the nature of the laser used as the vaporization source, the laser power selected, and, finally, the direet detection of cluster ions or the photoionization of neutral species drastically influence the relative proportion of met-cars to nanocrystals. This selectivity has been attributed to distinct mechanisms of cluster growth for met-cars and nanocrystals (Section 5.9.2.3). Laser-induced photodissociation of 3 X 3 X 3 nanocrystals M14C13+ (M = Ti, V) and larger clusters assumed to have a fee crystal structure has been reported by Pilgrim and Duncan. The titanium carbide cluster corresponding to the 1044-amu peak in the mass spectrum has been assigned to the 3 x 3 x 4 fee fragment in which one tita-... 

Mass spectrometry, which is the only technique that can be used to characterize met-cars and related metal-carbide clusters, implies that the detected clusters are ionized. This requirement opens a route to a variety of experimental procedures enabling insight to be gained into physical properties such as ionization energies, electron affinities, structure, and collective electronic properties such as thermionic electron emission and delayed atomic ion emission. 

In contrast to the soft ESI ion formation technique, LA/ionization of solids as a means to produce metal cluster ions is a high-energy process that can provide various types of species, generally with strong covalent bonds, such as in oxides or carbides, as opposed to the weaker bonding exhibited in species produced by ESI. Cluster ion chemistry studies with rare earths and actinides have developed a solid history, with metallofullerenes playing an important role. 

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