Cluster composition

The size distribution of the clusters produced in the cluster source is quite smooth, containing no information about the clusters except their composition. To obtain information about, for example, the relative stability of clusters, it is often useful to heat the clusters. Hot clusters will evaporate atoms and molecules, preferably until a more stable cluster composition is reached that resists further evaporation. This causes an increase in abundance of the particularly stable species (i.e., enhancing the corresponding peak in the mass spectrum, then commonly termed fragmentation spectrum ). Using sufficiently high laser fluences (=50 /iJ/mm ), the clusters can be heated and ionized simultaneously with one laser pulse. 

From the binding energies calculated for the different cluster compositions, we determined abundance mass spectra for heated CggLi clusters from a simple Monte Carlo simulation. Figure 11 shows the simulated mass spectra resulting from these calculations, including the Li and Cgo isotope distributions. The peaks at A = 12 and at x = 6 + n (where n is the cluster charge) observed in the experiment (Fig. 9) are well reproduced. For more details, see ref. [12]. 

Figure 6. Absorption spectra of spherical non-interacting nanoclusters embedded in no absorbing matrices (a) effect of the size for Ag nanoclusters in silica (b) effect of the matrix for R = 2.5 nm Au clusters (the refractive index n = and the position of the plasma resonance are reported for each considered matrix) (c) effect of the cluster composition for i = 5 nm noble-metal clusters (Ag, Au, Cu) in silica. (Reprinted from Ref [1], 2005, with permission from Italian Physical Society.)...

Localization Cluster type Position of the Al atom Cluster composition... 

Cluster Composition for Representative Phoenix Aerosol Sample... 

Recently, a second or alternative nitrogenase has been isolated from Azotobacter vinelandii (21) and Azotobacter chroococcum (22) that contains vanadium as opposed to molybdenum. The MoFe and VFe nitrogenase proteins from A. vinelandii (called Av and. 4vl , respectively) are known to have different polypeptide structures and it obviously of interest to know to what extent the cluster composition is conserved. Variable temperature MCD studies of the as isolated and thionine oxidized proteins provided a convenient means of addressing this question. 

The spectra of silver and gold nanoclusters are intense and distinct (Table 4). They are thus particularly suitable to detect the evolution of a cluster composition during the construction of a bimetallic cluster in mixed solution. The system studied by pulse radiolysis was the radiolytic reduction of a mixed solution of two monovalent ions, the cyano-silver and the cyano-gold ions Ag(CN)2 and Au(CN)2 (Fig- 7) [66]. Actually, the time-resolved observation demonstrated a two-step process. First, the atoms Ag and Au are readily formed after the pulse and coalesce into an alloyed oligomer. However, due to... 

The same workers (234) also studied the methanation behavior of bimetallic clusters of Ru/Ni and Ru/Cu in zeolite Y. Such clusters can be formed by metals, such as ruthenium and copper which are immiscible as bulk metals (235, 236). The turnover numbers versus bimetallic cluster composition are shown in Fig. 22. Dilution of ruthenium with copper clearly causes a marked decrease in specific activity. This decrease in activity is also accompanied by a decrease in methanation selectivity. This was attributed to an inhibiting effect of copper on the ruthenium hydrogenolysis activity. 

An important parameter for comparison with theory as well as for understanding many properties would be relative binding energies or stabilities. Unfortunately those are hard to assess in the gas phase. One of the few experiments to report thermodynamic binding energies between base pairs is the work by Yanson et al. in 1979, based on field ionization [25], Relative abundances of nucleobase clusters in supersonic beams are an unreliable measure of relative stability for a two reasons First, supersonic cooling is a non-equilibrium process and thus comparison with thermal populations is tenuous at best. Secondly, ionization probabilities may be a function of cluster composition. The latter is certainly the case for multi photon ionization, as will be discussed in detail below. 

Of course, there have to be differences in studies of pure and mixed clusters. In the latter, not only the optimal positions of the atoms in the cluster are to be found, but also the optimal distribution of the atom types on these positions. Therefore, additional evolutionary operators may be introduced that directly change this atom-type distribution without affecting the atom positions in space. For the resulting structures, additional questions can be asked How and how strongly do optimal structures change with cluster composition Do the different atom types prefer to mix or to segregate within the clusters ... 

It is interesting to note that the base displaced need not be a simple one, that is, dihydrogen is exclusively displaced from [HFe3(CO)9(/u-H)2BH] (equation 10). Indeed, a comparison of equations (10) and (11) demonstrates that cluster composition is important in determining the outcome of a Lewis base displacement reaction on a transition metal-main group element cluster. This implies that control of electronic structure via cluster element composition is a viable means of controlling reactivity. 

Table VIII. Five Midwestern Localities Cluster Composition Comparisons...

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