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  • American Institute of Physics (AIP)  (23)
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  • 1
    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.
    Type of Medium: Electronic Resource
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  • 2
    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.
    Type of Medium: Electronic Resource
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  • 3
    Electronic Resource
    Electronic Resource
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 87 (1987), S. 5238-5241 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Clusters of Ne atoms produced by adiabatic nozzle expansion have been studied with a high resolution double focusing sector field mass spectrometer. Analysis of the isotopic composition of these clusters (up to n=22) showed instead of the expected statistical distribution a pronounced shift to the heavier neon isotope 22Ne. The shift was found to correspond to an increase of the isotope ratio of 22Ne from 8.7% in the monomer to ∼12.5% in the clusters with n〉4 (enrichment factor of ∼1.5.)
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  • 4
    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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  • 5
    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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  • 6
    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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  • 7
    Electronic Resource
    Electronic Resource
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 96 (1992), S. 9241-9242 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Electron attachment to SF6 and SF6 clusters has been investigated in a molecular beam/electron impact ion source/mass spectrometer system. In accordance with recent theoretical predictions we were able to produce the superhalogen ion SF7− by electron attachment to SF6 clusters. SF7− has not been previously observed; its relative attachment cross section curve shows features more similar to that of F2− from SF6 than that of F− from SF6.
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  • 8
    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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  • 9
    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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  • 10
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Using recently measured accurate relative partial ionization cross section functions for production of the C60 fragment ions C+58 through C+44 by electron impact ionization, we have determined the respective binding energies BE(C+n–C2), with n=58,...,44, using a novel self-consistent procedure. Appearance energies were determined from ionization efficiency curves. Binding energies were calculated from the corresponding appearance energies with the help of the finite heat bath theory. Then using these binding energies we calculated with transition state theory (TST), the corresponding breakdown curves, and compared these calculated ones with the ones derived from the measured cross sections. The good agreement between these breakdown curves proves the consistency of this multistep calculation scheme. As the only free parameter in this procedure is the binding energy C+58–C2, we studied the influence of different transition states chosen in the determination of this binding energy via TST theory and iterative comparison with breakdown curve measurements. Based on this study we can conclude that extremely loose transition states can be confidently excluded, and that somewhat looser transition states than those used earlier result in an upward change of the binding energy of less than 10% yielding an upper limit for the binding energy C+58–C2 of approximately 7.6 eV. © 1996 American Institute of Physics.
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