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  • 1990-1994  (4)
  • 1991  (4)
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  • 1990-1994  (4)
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  • 1
    Electronic Resource
    Electronic Resource
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 94 (1991), S. 810-811 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Singly charged argon cluster ions produed by electron impact ionization of a neutral argon cluster beam are found to decay in the metastable time regime by a new mechanism leading to the ejection of "magic'' numbers of neutral argon atoms. The measured dependence of this new decay process on (i) the electron energy, (ii) cluster size and cluster properties, and (iii) the time since ion formation gives insight into the unique mechanism and the nature of this process. At a well-defined threshold energy of ca. 27 eV, the magic number loss mechanism occurs simultaneously with the well-known single monomer evaporation process which proceeds at all energies. Importantly, the new mechanism is the first known example of cluster ion metastability showing an exponential dependence on time, providing further evidence that the precursor parent cluster ion is produced in a specific energy state.
    Type of Medium: Electronic Resource
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  • 2
    ISSN: 1434-6079
    Keywords: 36.40
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Notes: Abstract Stoichiometric and non-stoichiometric, positive and negative oxygen cluster ions (n up to 70) have been produced in a crossed neutral beam/electron beam ion source. The abundance and stability of the ions formed have been analyzed with a double focussing sector field mass spectrometer in a series of experiments. Positive and negative ion mass spectra observed exhibit distinct abundance anomalies, however, at different cluster sizes. Abundance maxima and minima correlate with correspondingly small and large metastable fractions of (O2) n + and (O2) n − ions, respectively. (O2) n + ions may also lose up top=(n−1) monomers by collision induced dissociation with monotonously decreasing probability with increasingp. Metastable fractions determined for (O2) n − ions produced with appr. zero eV electrons are in general larger than those for ions produced with appr. 7 eV electrons. (O2) n − ions are also observed to decay via autodetachment, with lifetimes increasing with increasing cluster size. Finally, here we were able to prove that an apparent loss of the monomer fragment O (and higher homologues) observed in the metastable time regime is due to ordinary metastable monomer evaporation in the acceleration region. Moreover, we will also present here some new data and interpretation concerning the electron attachment cross section function for O2 clusters.
    Type of Medium: Electronic Resource
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  • 3
    ISSN: 1434-6079
    Keywords: 36.40
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Notes: Abstract Production and stability of Ar n +* ions (withn up to 420) formed by electron impact ionization of a supersonic Ar cluster beam were investigated with a double focussing sector field mass spectrometer. The present study confirms previous magic number determinations up to the 4th icosahedral shell. A systematic study of metastable dissociations (monomer evaporation, magic number evaporation) for singly charged cluster ions as a function of cluster size, internal excitation energy and time elapsed since ion formation gives new insight into the ionization process and subsequent reactions of the ions formed. At a well-defined threshold energy ofca. 28 eV, the magic number loss mechanism occurs simultaneously with the well known single monomer evaporation process which proceeds at all energies. The new mechanism is the first known example of cluster ion metastability showing an exponential dependence on time, providing further evidence that the precursor parent cluster ion is produced in a specific energy state.
    Type of Medium: Electronic Resource
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  • 4
    Electronic Resource
    Electronic Resource
    New York, NY : Wiley-Blackwell
    Rapid Communications in Mass Spectrometry 5 (1991), S. 51-57 
    ISSN: 0951-4198
    Keywords: Chemistry ; Analytical Chemistry and Spectroscopy
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Physics
    Notes: A review of mass spectrometric methods used to study different kinds of clusters is presented. These methods have proved to be indispensable in the cluster field owing to their great flexibility, sensitivity and wide dynamic range. This reviews presents new mass spectrometric methods and techniques such as quadrupole analysers and RF lenses, different ion-trap geometries, time-of-flight (TOF) methods including the reflectron geometry). A variety of ionization methods, both for the production of positive and negative ions, with different amounts of released excess energy, are discussed in detail. Special attention is given to different degree of post-ionization fragmentaqtion and metastability of ions produced by the respective ionization methods. The ability to follow the metastable decay processes in rather different time-windows is also one of the salient features of the mass spectrometric methods reviewed here, involving both classical and new geometries. Detection of very large cluster ions by conventional methods is not always effective, and alternative methods like post-acceleration and using a conversion dynode in front of the multiplier will be included here. Moreover, mass spectrometric methods offer the possibility of producing beams of mass-selected cluster ions necessary for the study of a whole line of process like optical spectrometry, photodetachment, photofragmentation, photoionization, electron beam ionization, and charge-exchange processes. Examples of these processes are also discussed in this review.
    Additional Material: 6 Ill.
    Type of Medium: Electronic Resource
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