227 research outputs found
Evidence for two-electron processes in the mutual neutralization of O- with O+ and N+ at Subthermal Collision Energies
We have measured total absolute cross sections for the Mutual Neutralization
(MN) of O- with O+/N+. A fine resolution (of about 50 meV) in the kinetic
energy spectra of the product neutral atoms allows unique identification of the
atomic states participating in the mutual neutralization process. Cross
sections and branching ratios have also been calculated down to 1 meV
center-of-mass collision energy for these two systems with a multi-channel
Landau-Zener model and an asymptotic method for the ionic-covalent coupling
matrix elements. The importance of two-electron processes in one-electron
transfer is demonstrated by the dominant contribution of a core-excited
configuration of the nitrogen atom in N+ + O- collisions. This effect is
partially accounted for by introducing configuration mixing in the evaluation
of coupling matrix elements.Comment: 5 pages, 4 figure
Lifetimes of C-60(2-) and C-70(2-) dianions in a storage ring
C-60(2-) and C-70(2-) dianions have been produced by electrospray of the monoanions and subsequent electron pickup in a Na vapor cell. The dianions were stored in an electrostatic ring and their decay by electron emission was measured up to 1 s after injection. While C-70(2-) ions are stable on this time scale, except for a small fraction of the ions which have been excited by gas collisions, most of the C-60(2-) ions decay on a millisecond time scale, with a lifetime depending strongly on their internal temperature. The results can be modeled as decay by electron tunneling through a Coulomb barrier, mainly from thermally populated triplet states about 120 meV above a singlet ground state. At times longer than about 100 ms, the absorption of blackbody radiation plays an important role for the decay of initially cold ions. The tunneling rates obtained from the modeling, combined with WKB estimates of the barrier penetration, give a ground-state energy 200 +/- 30 meV above the energy of the monoanion plus a free electron and a ground-state lifetime of the order of 20 s. (c) 2006 American Institute of Physics
Failure of hydrogenation in protecting polycyclic aromatic hydrocarbons from fragmentation
A recent study of soft X-ray absorption in native and hydrogenated coronene
cations, CH , led to the conclusion that additional
hydrogen atoms protect (interstellar) Polycyclic Aromatic Hydrocarbon (PAH)
molecules from fragmentation [Reitsma et al., Phys. Rev. Lett. 113, 053002
(2014)]. The present experiment with collisions between fast (30-200 eV) He
atoms and pyrene (CH, , 6, and 16) and simulations
without reference to the excitation method suggests the opposite. We find that
the absolute carbon-backbone fragmentation cross section does not decrease but
increases with the degree of hydrogenation for pyrene molecules.Comment: 10 pages, 5 figure
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