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  • 1
    ISSN: 1434-6079
    Keywords: PACS: 36.40.Qv Stability and fragmentation of clusters – 81.05.Tp Fullerenes and related materials; diamonds, graphite
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Notes: Abstract. Spontaneous (metastable) and surface-induced decay reactions for multiply charged fullerene ions, investigated recently in Innsbruck, are reviewed. Results discussed include the mechanisms and energetics of C2 evaporation and charge separation reactions, the secondary electron emission upon impact on gold surfaces, and surface-induced reactions of singly and multiply charged fullerene ions.
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  • 2
    ISSN: 1434-6079
    Keywords: PACS: 36.40.-c Atomic and molecular clusters – 34.80.Lx Electron–ion recombination and electron attachment
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Notes: Abstract. The formation of both positively and negatively charged ions after interaction of electrons with different cluster beams is investigated, by the use of highly monochomatized electron beams. In the case of the electron attachment to neutral NO clusters, the formation of the monomer ion NO- could be observed for the first time. The only explanation for the creation of such an ion is an intracluster reaction. In the case of carbon monoxide clusters, the appearance energies were determined with high accuracy. From these data, we derived the binding energy of the cluster dimer. Our results are in good agreement with photoionization studies.
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  • 3
    ISSN: 1572-9540
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Notes: Abstract The formation of fragment ions from C60 due to electron impact ionization leading to a bimodal fragmentation pattern with a minimum around the cluster size of 30 is investigated with help of mass spectrometric techniques. Based on a detailed analysis of the energetics and kinetics of the singly- and multiply-charged fragment ions by two-sector field mass spectrometry (MIKE-scans) we can conclude that on the one hand some of the dissociation reactions proceed via evaporation of an intact C6 + or C4 + unit (and not via the sequential loss of smaller molecular carbon units). On the other hand, measured appearance energies and the direct observation of sequential C2-loss reactions confirm that the sequential loss of neutral C2 units is the dominant production mechanism for Cn z+ fragment ions with sizes 40 〈 n 〈 60.
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  • 4
    ISSN: 1572-9540
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Notes: Abstract Highly-charged fullerene ions C 60 z+ and C 70 z+ with charge states up to z=7 have been produced in an electron impact ion source of a two sector field mass spectrometer by using ion source operating conditions similar to those used in EBIT sources. The stability of these ions was investigated quantitatively in the two field free regions of the mass spectrometer. It was found that besides C2 evaporation the dominant fission process for ions with charges larger than +2 is the loss of a charged C 2 + unit via a super-asymmetric charge separation reaction C 60 z+ → C 58 (z−1)+ +C 2 + and C 70 z+ → C 68 (z−1)+ +C 2 + , respectively. The most important finding from these studies is that this super-asymmetric dissociation reaction proceeds via a three stage reaction sequence involving an electron transfer reaction at the second stage between a receding C2 unit and the remaining highly-charged fullerene cage.
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  • 5
    ISSN: 1434-6079
    Keywords: PACS. 36.40.Ei Phase transitions in clusters - 05.70.Fh Phase transitions: general aspects - 36.40.Qv Stability and fragmentation of clusters
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Notes: Abstract: We report the first account of a cluster fragmentation study involving high energy cluster-cluster collisions in which all the fragments of each collision occurring in the experiment are mass analyzed on an event by event basis. This allows an unbiased look at the true nature of fragmentation including a statistical analysis in terms of fluctuations in the fragment size distribution.
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  • 6
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Using a high resolution two sector field mass spectrometer of reversed geometry we have measured metastable fractions and mass-analyzed ion kinetic energy peaks for metastable decay reactions involving monomer evaporation via (O2)n+*→(O2)n−1++O2. Both, average kinetic energy release 〈KER〉 data derived from the peak shapes and the time dependence of the metastable fractions show a characteristic dependence on cluster size indicating a change in the metastable fragmentation mechanism when going from the dimer to the dekamer ion. Moreover, the 〈KER〉 data contain information about the transition state temperature and thus one can use finite heat bath theory to calculate the binding energies of the decaying cluster ions. The results obtained are in fair agreement with (i) previous results based on gas phase ion equilibria measurements and with (ii) the corresponding bulk value. © 2002 American Institute of Physics.
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  • 7
    Electronic Resource
    Electronic Resource
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 114 (2001), S. 9875-9881 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Electron impact ionization of small singly charged hydrocarbon ions was applied to produce and investigate multiply charged molecular ions. The stability of triply charged hydrocarbon ions depends strongly on the number of hydrogen atoms of the molecule. C4Hj3+ were observed only for molecules that contain more than 2 hydrogen atoms. Parallel, highly correlated ab initio calculations were performed for these molecular ions. The binding energies obtained by these computations agree well with the experimental findings. Moreover, there is quantitative agreement between the experiment and the calculations on the kinetic energy of the fragment ions upon Coulomb explosion. © 2001 American Institute of Physics.
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  • 8
    Electronic Resource
    Electronic Resource
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 111 (1999), S. 3548-3558 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Highly monochromatized electrons (with energy distributions of less than 30 meV FWHM) are used in a crossed beam experiments to investigate electron attachment to oxygen clusters (O2)n at electron energies from approximately zero eV up to several eV. At energies close to zero the attachment cross section for the reaction (O2)n+e→(O2)m− (for m=1, 2, and 3) rises strongly with decreasing electron energy compatible with s-wave electron capture to (O2)n. Peaks in the oxygen attachment cross sections present at higher energies ((approximate)80 meV, 193 meV, 302 meV) can be ascribed to vibrational levels of the anion populated by attachment of an electron to a single oxygen molecule within the target cluster via a direct Franck–Condon transition from the ground vibrational state v=0 to a vibrational excited state v′=7,8,9,... of the anion produced. The vibrational structures observed here for the first time can be quantitatively accounted for by model calculations using a microscopic model to examine the attachment of an electron to an oxygen molecule inside a cluster. This involves (i) molecular dynamics simulations to calculate the structure of neutral clusters prior to the attachment process and (ii) calculation of the solvation energy of an oxygen anion in the cluster from the electrostatic polarization of the molecules of the cluster. The occurrence of this polarization energy at the surface of larger clusters explains the appearance of an s-wave capturing cross section at 0 eV and the slightly smaller spacings (compared to the monomer case) between the peaks at finite energy, as observed experimentally. The relative transition probabilities from the ground state of the neutral oxygen molecule to the different vibrational levels of the anion are obtained by calculating the corresponding Franck–Condon factors thereby resulting in a reasonable theoretical fit to the observed yields of negatively charged oxygen molecules and clusters. © 1999 American Institute of Physics.
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  • 9
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: We have obtained direct mass spectrometric evidence that fullerene ions C60z+ (z=1, 2, or 3) and C58z+(z=1,2) undergo unimolecular dissociation by sequential emission of twoC2 units, on a time scale of 10−5 s. Moreover, a comparison of experimental and theoretical breakdown graphs reveals that unimolecular formation of C56+ from the C60+ parent ion within a given observational time window is dominated by successive loss of C2; direct C4 loss does not contribute significantly. This conclusion is not affected by uncertainties in our knowledge of the energetics of C2 vs C4 loss. © 1997 American Institute of Physics.
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  • 10
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Using a crossed electron-molecule beam ion source in combination with a quadrupole mass spectrometer we have studied the electron energy dependence of the dissociative attachment process CHCl3+e→Cl− at electron energies from about 0 to 2 eV and in a target gas temperature range of about 300–430 K. The energy resolution and working conditions of this newly constructed crossed beams machine have been characterized using CCl4 as a test and calbrant gas. Utilizing the improved energy resolution of the present experimental setup (which allows measurements with FWHM energy spreads down to below 5 meV) it was possible to determine the accurate shape and magnitude of the cross section function in the low-energy range. This leads to the conclusion that between an electron energy of about 20 and 130 meV the reaction proceeds via deBroglie s-wave capture, whereas at higher energy (above about 0.4 eV) autodetachment plays a significant role. Moreover, the present measurements allow us to clarify previously reported differences in the absolute cross section, the number of peaks and in the energy position of these peaks. Finally, by analyzing the measured strong temperature dependence of the cross section close to zero electron energy the activation barrier for this dissociative attachment was determined to be 110±20 meV in good agreement with thermochemical data from swarm experiments performed under thermal equilibrium. Taking into account the present results it is also possible to discuss the mechanism for the existence of the second peak. © 1997 American Institute of Physics.
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