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  • 1
    ISSN: 1434-6079
    Keywords: 36.40 ; 07.75
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Notes: Abstract Clusters of Ar, Kr, Xe, N2, O2, CO2, SO2 and NH3 formed by supersonic nozzle expansion have been studied by electron impact ionization mass spectrometry (up to 15000 amu). Besides mass spectra of singly charged ions showing the characteristic anomalous distributions, we have in particular investigated the properties of multiply charged cluster ions. Critical appearance sizes of doubly and triply charged cluster ions, n2 and n3 respectively, found in the present study confirm recent theoretical predictions about n3/n2 and their dependence on the properties of the cluster constituents. The appearance energies of multiply charged cluster ions determined are shifted way below the appearance energies of the respective monomer ions. These huge red shifts together with the observed linear threshold laws and large maximum ionization efficiencies indicate that multiply charged cluster ions are produced by sequential single ionization events of one incoming electron at different cluster sites. Furthermore, we have also obtained for the first time clear evidence that (for electron energies above the appearance energy of doubly charged ions) an appreciable amount of singly charged (also fragment) ions is produced via Coulomb explosion of unstable doubly charged ions in the ion source.
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  • 2
    ISSN: 1434-6079
    Keywords: 36.40 ; 34.80.G
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Notes: Abstract Electron attachment and electron impact ionization of SF6 clusters have been investigated quantitatively in a molecular beam/electron ion source/mass spectrometer system as a function of electron energy E (0≤E≤180 eV) and as a function of cluster size.
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  • 3
    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.
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  • 4
    ISSN: 1572-9486
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Notes: Abstract The 4.4eV dissociative electron attachment peak in CO2 was reinvestigated paying particular attention to (i) its structure associated with vibrational excitation and to (ii) the temperature dependence of the onset. For this purpose we have used two specially designed crossed beams machines, one having a trochoidal electron analyzer (TEM) and one with a hemispherical electron analyzer (HEM) for the production of the highly monochromatized electron beams (with FWHM's down to 5 and 50meV, respectively). The present results confirm the earlier findings in (i) interpreting the dominant structures of the 4.4eV peak as being due to vibrationally excited states of CO in the reaction CO2 + e → O-+ CO and (ii) assigning the much weaker and narrower structures to the intermediate CO 2 - . In the temperature range between 300 and 245 K almost no temperature dependence of the onset can be seen in the present study. In comparison to NO and CO where the onset is vertical the CO2 threshold behavior is less steep indicating that in the CO2 case a different type of transition must be responsible for the onset of the O− production. Besides DA to CO2 we have for comparison and calibration purposes investigated also DA to CO and NO.
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  • 5
    ISSN: 1434-6079
    Keywords: 33.80.G
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Notes: Abstract We have observed strong photodissociation (using visible laser light) of neutral van der Waals clusters (Ar, N2, O2, CO2, SO2, NH3) produced by supersonic expansion and detected by electron ionization/mass spectrometer. Several tests were performed, all of them supporting this surprising discovery. We suggest that Raman induced photodissociation (RIP) is responsible for this phenomenon. This first observation of Raman induced photodissociation provides a new technique for the study of neutral van der Waals clusters.
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  • 6
    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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  • 7
    ISSN: 1434-6079
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Notes: Abstract Recently we have observed [1–4] photodissociation of neutral van der Waals clusters induced by visible laser light using coaxial interaction between the beams. Because of possible artifacts we have repeated here this experiment in a crossed beam configuration. The present results confirm the occurrence of this photodissociation effect. We suggest tentatively that excitation via Raman scattering is the origin of this phenomenon.
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  • 8
    Electronic Resource
    Electronic Resource
    Springer
    The European physical journal 6 (1987), S. 351-362 
    ISSN: 1434-6079
    Keywords: 36.40 ; 35.20
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Notes: Abstract Electron attachment to SO2 clusters formed by nozzle expansion was investigated (n up to 8) in a molecular-beam electron-ionization mass-spectrometer system. Electron ionization of SO2 clusters was also studied (n up to 18) showing no pronounced structure in the mass spectra and no strong dependence on electron energy, the dominant positive ion being the (SO2) n + series. Also present but less abundant are the fragment ion series (SO2) n SO+, (SO2) n S+ and (SO2) n O+ with decreasing intensities in that order. The dominant negative ion is (SO2) n − . The homologous series (SO2) n O− and (SO2) n SO− are less abundant, the series (SO2) n S− has not been observed at all (except SO2·S− produced in the background gas via secondary processes). The negative ion mass spectra show a strong dependence on electron energy due to a rich resonance like structure of the attachment cross sections involved. These attachment cross sections have been determined (up to 40 eV) and they show significant differences for the different homologous series. The most striking feature is that for instance (SO2) n SO− ions do not show any signal at the first main resonance of SO− from SO2. Likewise (SO2) n O− ions only show a strongly diminished signal at the first main resonance of O− from SO2. This is in contrast to results in O2, CO2 and N2O. Conversely, (SO2) n − ions show — besides peaks at the position of the first and second O− resonance — additional resonances below and above these peaks. In addition, (SO2) 5 − and larger ones show a zero energy peak consisting of stoichiometric SO2 cluster ions similar to observations in O2, CO2 and H2O. The attachment cross section of S− from SO2 has been found to show an additional previously undetected peak at ∼1 eV. Moreover, the present study revealed the existence of a S n − (n up to 8) series being produced with nearly zero energy electrons via volume and surface processes in and around the ion source.
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  • 9
    Electronic Resource
    Electronic Resource
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 90 (1989), S. 1288-1289 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: We report the first well resolved SO2 cluster ion spectrum showing doubly charged parent and fragment ions. The critical appearance size deduced for doubly charged SO2 clusters is n2=21 and for triply charged SO2 clusters n3=49. This result is compared with theoretical predictions.
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  • 10
    Electronic Resource
    Electronic Resource
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 89 (1988), S. 295-301 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Metastable decay of Ar+@B|3 produced by electron impact ionization of a supersonic Ar cluster beam has been investigated qualitatively and quantitatively with a double focusing sector field mass spectrometer. We present direct experimental proof that the observed metastable decay of Ar+*3 into Ar+ in the microsecond time regime proceeds predominantly via a sequential decay series Ar+@B|3→k3,2Ar+*2→ k2,1Ar+ involving the evaporation of a single monomer in each of these successive decays. The metastable decay rates deduced are very different, i.e., k3,2 =45 s−1 and k2,1 =680 s−1 at an ion acceleration voltage of 3 kV. Moreover, the metastable decay rates of Ar+@B|3 exhibit a strong dependence on time elapsed since electron impact production of the Ar+3, and on ion production parameters (i.e., size and internal excitation energy of the neutral precursor). For instance photodissociation (with visible laser light) of the neutral cluster beam leads to a dramatic increase of the observed metastable reaction rules yielding values of up to 100 000 3−1 in the μs time regime.
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