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  • American Institute of Physics (AIP)  (10)
  • 2000-2004  (3)
  • 1995-1999  (7)
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  • 1
    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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  • 2
    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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  • 3
    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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  • 4
    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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  • 5
    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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  • 6
    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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  • 7
    Electronic Resource
    Electronic Resource
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 108 (1998), S. 963-970 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: We used mass spectrometric techniques in conjunction with the ion deflection method to determine the kinetic energies of Cn+ fragment ions (58≥n≥4) produced by controlled electron impact on C60 under single collision conditions. The recorded ion beam profiles for the various fragment ions were analyzed using two independent methods. One method extracts the average kinetic energy of the fragment ion under study from the measured half-width of the ion beam profile (half-width method), whereas the second method analyzes the entire measured beam profile (profile method). For each Cn+ fragment ion, the kinetic energies obtained were interpreted assuming two possible formation pathways, (i) the fission of the excited parent C60+ ion in a single-step, two-fragment break-up, C60+→Cn++C60−n and (ii) the sequential decay of excited C60+ into Cn+ via the successive removal of Cm units (m=1, 2, or 3). Both the half-width method and the profile method yield very similar results for the kinetic energies of the Cn+ fragment ions for both formation mechanisms, viz., an average kinetic energy of roughly 0.45 eV in the case of the single-step fission and a linearly increasing average kinetic energy from 0.43 eV for the formation of C58+ to roughly 12 eV for the formation of C4+ in case of the sequential decay mechanism (in this case an essentially constant energy of also about 0.45 eV was obtained for the last fragmentation step in each case, which was confirmed by a third method using a fitting procedure). These results will be discussed in the context of the most likely fragmentation mechanism leading to the formation of the various fragment ions. © 1998 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 108 (1998), S. 990-1000 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Using high sensitivity two sector field mass spectrometric techniques (in particular MIKE scans) we have studied quantitatively (and systematically) the energetics of the superasymmetric spontaneous decay reactions (involving either C2+ or C4+ loss) of triply, quadruply, quintuply, and sextuply charged carbon clusters ions Cnz+ in the size range from n=36 up to n=70. From the kinetic energy release data determined, the apparent intercharge distance has been derived using different models including the simple point charges model, the movable charges model and the charged conducting sphere model. As in earlier but less extensive studies the intercharge distance obtained is for all three models used larger than the cage radius of the respective precursor fullerene ion. It is shown that this and other experimental results are only compatible with the recently suggested auto charge transfer (ACT) reaction as the decay mechanism responsible for the superasymmetric charge separation reactions, whereas two other conceivable decay mechanisms (ball-chain-propagation and decay of charged conducting liquid sphere) are not consistent with all of the experimental fingerprints observed. © 1998 American Institute of Physics.
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  • 9
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Electron impact ionization of C60 and C70 was studied using a molecular/electron beam ion source in combination with a two sector field mass spectrometer operated in the ion beam deflection mode. Relative partial ionization cross sections for the production of singly and multiply charged parent ions (up to charge state z=4) and fragment ions (down to C+44 in the case of C60 and down to C2+50 in the case of C70) were determined from threshold up to 1000 eV electron energy. Absolute partial and total ionization cross sections are obtained using a novel approach for the absolute calibration involving an intercomparison of the cation with the anion yield. The results obtained reveal not only an anomalous large parent ion cross section as compared to other ionization channels [e.g., σ(C+60/C60) is more than a factor of 30 larger than σ(C+58/C60)] but also anomalies for the production of multiply charged parent and fragment ions. For instance, the maximum cross section for the formation of C2+60 amounts to 30% of the maximum C+60 cross section and that of C2+70 to about 50% of C+70. Moreover, for all fragment ions, the formation of the doubly charged fragment ions has a larger cross section than that of the respective singly charged fragment ion. These peculiar features of the kinetics of electron impact ionization of C60 and C70 are related to the specific electronic and geometric structures of these fullerenes. The present absolute cross-section data for the summed up partial cross sections are in good agreement with a recent semiclassical calculation from our laboratory. © 1996 American Institute of Physics.
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  • 10
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Appearance energies of all parent ions and several fragment ions produced by electron impact ionization of the isotope systems H2/D2, H2O/D2O, and C6H6/C6D6 were determined with high precision using a dedicated high-resolution electron impact ionization mass spectrometer. The determination of the appearance energies from scans of the ion signal as a function of electron energy in the near-threshold region of each ion utilized a fitting and analysis procedure that has recently been successfully applied to the determination of appearance energies of singly and multiply charged rare-gas ions and several molecular ions and cluster ions. The experimentally determined appearance energies are in good agreement (i) with theoretical calculations that we carried out using standard quantum chemistry codes and (ii) with appearance energy values listed in standard reference data tables (to the extent that tabulated values are available). We find isotope shifts for all three systems ranging from a few meV for the parent ions to a few hundred meV for the fragment ions. The deuterated species always have the higher appearance energy. The present results for H2O/D2O and C6H6/C6D6 do not confirm the larger isotope effects that were reported earlier by Snegursky and Zavilopulo [Nucl. Instrum. Meth. Phys. Res. B 126, 301 (1997)]. © 2002 American Institute of Physics.
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