- Translational energy dependence of reaction mechanism: Xe+ + CH4->XeH+ + CH3
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The dynamics of the exoergic ion-molecule reaction Xe+(CH4,CH3)XeH+ were studied by chemical accelerator techniques over the relative translational energy range 0.2 to 8 eV.Results of the kinematic measurements are reported as scattering intensity contour maps in Cartesian velocity space.Center-of-mass angular and energy distributions, derived from these maps, provide information on the reaction mechanism and on the partitioning of available energy between internal and translational modes in the products.The results suggest that reaction proceeds via the formation of a long-lived complex at low collision energies (below 0.5 eV) and by a direct mechanism approaching spectator stripping at higher energies.
- Miller, G. D.,Strattan, L. W.,Cole, C. L.,Hierl, P. M.
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- Electron-impact excitation of autoionizing states in xenon
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High-sensitivity single-ionization efficiency functions of Xe have been measured using a computer-controlled beam of electrons monoenergetic to ca. 45 meV FWHM.Well-defined structure detected in the energy range 12.2-13.5 eV is attributed to the excitation of neutral Xe Rydberg states converging on XE+(2P1/2) followed by autoionization.Some of the structure appears to correspond to parity forbidden s-p' transitions.A sharp increase in ionization is observed at the 2P1/2 threshold.
- Mathur, Deepak,Frost, David C.
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- Role of impact parameter in branching reactions: Chemical accelerator studies of the reaction Xe+ + CH4->XeCH3+ + H
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Integral reaction cross sections and product velocity distributions have been measured for the ion-molecule reaction Xe+(CH4,H)XeCH3+ over the relative reactant translational energy range of 0.7-5.5 eV by chemical accelerator techniques.The kinematic results indicate that reaction proceeds in a direct manner by a rebound mechanism over the energy range studied, suggesting that this substitution reaction occurs predominantly in small impact parameter collisions.This finding contrasts with the results obtained for the competing reaction, Xe+(CH4,CH3)XeH+, where the strong forward scattering of the XeH+ product indicates that H-atom abstraction occurs primarily in large impact parameter collisions.
- Miller, G. D.,Strattan, L. W.,Hierl, P. M.
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- The decay dynamics of photoexcited rare gas cluster ions
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The kinetic energies of fast neutrals ejected from photoexcited rare gas cluster ions have been measured for the following systems: Arn+, Krn+, Xen+ at two photon wavelengths: 355 and 532 nm, and for n in the range 2-19. New data are presented for xenon at both wavelengths, and for argon and krypton cluster ions at 355 nm. For argon and krypton cluster ions at 532 nm, new data have been recorded which are more accurate than those presented previously. A Monte Carlo model of the experiment has been used to simulate the kinetic energy releases and also to investigate variations in the scattering anisotropy parameter (β) as a function of photon energy and cluster composition and size. Significant fluctuations in β are observed, and these are attributed to a combination of structural variation and changes to the nature of the central chromophore. For small cluster ions the kinetic energy release data show evidence of being influenced by the final spin-orbit state of the atomic ion. Overall, there is a gradual decline in kinetic energy release as a function of increasing cluster size; however, there are marked variations within this trend. For all three rare gas systems the results show that the primary response to photoexcitation is the ejection of a single atom with a high kinetic energy on a time scale that is short compared with the rotational period of a cluster.
- Jones,Jukes,Stace
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p. 959 - 968
(2007/10/03)
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- Collisions of rare gas ions with C60: Endohedral formation, energy transfer, and scattering dynamics
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Scattering of rare gas cations from C60 has been studied in a guided-beam tandem mass spectrometer. Charge transfer (CT) is observed to be the dominant channel over the collision energy range from O to 100 eV, but dissociative CT and endohedral complex formation are significant at high collision energies. The threshold energies for endohedral penetration are found to be proportional to rare gas atom size. Our CT and dissociative CT data allow us to make several conclusions about the nature of energy transfer in rare gas-fullerene collisions. Surprisingly, the conclusion is that the energy transfer distribution must be sharply bimodal, with ~85% of collisions resulting in little collision-to-internal energy transfer, and ~15% of collisions being essentially 100% inelastic. The results indicate that the dissociative CT and endocomplex formation channels are closely related.
- Basir, Yousef,Anderson, Scott L.
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p. 8370 - 8379
(2007/10/03)
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- Absolute cross sections for electron-impact ionization and dissociative ionization of the SiF free radical
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Absolute cross sections for electron-impact ionization of the SiF free radical from threshold to 200 eV are presented for formation of the parent SiF+ ion and the fragment Si+ and F+ ions.A fast beam of SiF is prepared by charge transfer neutralization of an SiF+ beam.The radicals form in the ground electronic state and predominantly in their ground vibrational state, as shown by agreement of the measured ionization threshold with the ionization potential.The absolute cross section for SiF -> SiF+ at 70 eV is 3.90 +/- 0.32 Angstroem2.The ratio of cross sections for formation of Si+ to that for SiF+ at 70 eV is 0.528 +/- 0.024; the ratio for formation of F+ to that of SiF+ is 0.060 +/- 0.008.The observed threshold energy for Si+ formation indicates the importance of ion pair formation SiF -> Si+ + F-.Breaks in the cross section at 14.3 and 17 eV are assigned as dissociative ionization thresholds.
- Hayes, Todd R.,Wetzel, Robert C.,Baiocchi, Frank A.,Freund, Robert S.
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p. 823 - 829
(2007/10/02)
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