Ion Cluster Distributions

In their LDI-MS investigation of ZnO and ZnSe deposits, Kukreja et al. demonstrated the critical importance of laser fluence on the resulting LDI mass spectra due to changes in the plume composition and ion cluster distribution [52]. In the positive-ion-generating mode and at a laser fluence of 100 J/m the LDI mass spectrum of a polycrystalline commercial sample of ZnSe consisted only of signals due to series of clusters of silver (an impurity), selenium, and silver-selenium species. At a laser fluence of 700 J/m the mass spectrum was domi-... 

Studying the formation of cluster ions and their ensuing ionization dynamics provides methods of ascertaining their thermodynamical properties, and in favorable cases, evidence of structures and the origins of magic numbers which appear in cluster distributions. 

Other studies conducted on mixed protonated clusters of ammonia bound with TMA showed that the ion intensity distributions of (NH3)n(TMA)mH+191 display local maxima at (n,m) = (1,4), (2,3), (2,6), (3,2), and (3,8). Observation that the maximum ion intensity occurs at (n,m) = (1,4), (2,3), and (3,2) indicates that a solvation shell is formed around the NHJ ion with four ligands of any combination of ammonia and TMA molecules. In the situation where the maximum of the ion intensity occurs at (n,m) = (2,6) and (3,8), the experimental results suggest that another solvation shell forms which contains the core ions [H3N-H-NH3]+ (with six available hydrogen-bonding sites) and [H3N-H(NH2)H-NH3]+ (with eight available hydrogen-bonding sites). The observed metastable unimolecular decomposition processes support the above solvation model. 

Ceynowa performed electron microscopic studies of 60—80 nm thick microtomed Nafion 125 membranes that were converted, for the purpose of affecting electron density contrast, to the Pb2+ form, and all of the excess cations and co-ions were removed. It is the heavy metal that provides electron density contrast between the phase in which it resides and the surrounding phase. These membranes were then exposed to ethanol and 1,2-epoxypropane, although these solvents would not have remained in the samples under the vacuum in the microscope column. The micrographs consisted of uniformly distributed points that were presumed to be ion clusters that were 3—6 nm in diameter. 

Figure 11. The ion cluster size distribution obtained from computer simulations of the charged and dipolar hard sphere mixture at several states half charge 1 Molar (A) fully charged, 1 Molar (B) fully charged, 0.4 Molar (C) and half charge, 0.4 Molar (D).

The mechanisms of ion formation are usually studied with measurements of mass separated ion energy distributions using a magnetic sector mass spectrometer.144,145 For a liquid metal ion source, both atomic ions and cluster ions of all sizes are emitted. If the total ion current is large, neutral atoms and small droplets may also be emitted. There is little question that most of the atomic ions in a liquid metal ion source at low... 

While intermediate ions are continuously produced by condensation onto small charged clusters, they are also constantly neutralized by ions of opposite charge, and are scavenged by ambient particles. These competing processes determine the initial rise and subsequent decay in the number of intermediate ions (Fig. 2). Before 14 30, the ion sizes and total number steadily increase. However, after 14 30, while the ion size still tends to increase slowly, the number decreases sharply due to neutralization by small ions. Corresponding to this evolution in the intermediate ion size distribution (and... 

This classification scheme applies to positive residual ions, and we are interested in neutral clusters. Bowers group (M. T. Bowers, personal communication) has also studied residual anions and find the essentially the same structural families but with quite different relative concentrations for a given mass and different ranges of cluster size where given families are important. Previous experience (O Brien et al. 1986) indicates that the cluster distributions seen with residual cations is more similar to that of the neutrals than is the residual anion distribution. Thus the use of cation... 

Whilst the rate coefficients for many binary and ternary negative ion-molecule reactions have been acquired recently, predominantly using the FA technique (see the data compilation of Albritton115 ), many more are required if the important paths in the synthesis of the observed negative ion clusters are to be identified. Product distributions have been studied even less for negative ion-molecule reactions, principally because of experimental difficulties, yet more than one product channel is accessible in several atmospherically important reactions73, for example,... 

Ions and ionized clusters ejected from a surface during ion bombardment are detected with a mass spectrometer. Surface chemical composition and some information on bonding can be extracted from SIMS ion fragment distributions. 

In the early years, there was some controversy regarding whether ionization of neutral clusters was accompanied by substantial evaporation of monomers from the cluster ions or if the cluster ion distribution reflects the neutral cluster distribution. Some groups concluded from their data that the CMS was a reflection of the neutral cluster distribution, that is, ionization of neutral clusters did not lead to significant fragmentation (Ding and Hesslich 1983 Echt et al. 1982 Sattler... 

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