| 1. |
- Denifl, S, et al.
(författare)
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Ionization of doped helium nanodroplets : Complexes of C-60 with water clusters
- 2010
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Ingår i: Journal of Chemical Physics. - : American Institute of Physics. - 0021-9606 .- 1089-7690. ; 132:23, s. 234307-
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Tidskriftsartikel (refereegranskat)abstract
- Water clusters are known to undergo an autoprotonation reaction upon ionization by photons or electron impact, resulting in the formation of (H2O)(n)H3O+. Ejection of OH cannot be quenched by near-threshold ionization; it is only partly quenched when clusters are complexed with inert gas atoms. Mass spectra recorded by electron ionization of water-doped helium droplets show that the helium matrix also fails to quench OH loss. The situation changes drastically when helium droplets are codoped with C-60. Charged C-60-water complexes are predominantly unprotonated; C-60(H2O)(4)(+) and (C-60)(2)(H2O)(4)(+) appear with enhanced abundance. Another intense ion series is due to C-60(H2O)(n)OH+; dehydrogenation is proposed to be initiated by charge transfer between the primary He+ ion and C-60. The resulting electronically excited C-60(+)* leads to the formation of a doubly charged C-60-water complex either via emission of an Auger electron from C-60(+)*, or internal Penning ionization of the attached water complex, followed by charge separation within {C-60(H2O)(n)}(2+). This mechanism would also explain previous observations of dehydrogenation reactions in doped helium droplets. Mass-analyzed ion kinetic energy scans reveal spontaneous (unimolecular) dissociation of C-60(H2O)(n)(+). In addition to the loss of single water molecules, a prominent reaction channel yields bare C-60(+) for sizes n=3, 4, or 6. Ab initio Hartree-Fock calculations for C-60-water complexes reveal negligible charge transfer within neutral complexes. Cationic complexes are well described as water clusters weakly bound to C-60(+). For n=3, 4, or 6, fissionlike desorption of the entire water complex from C-60(H2O)(n)(+) energetically competes with the evaporation of a single water molecule. (C) 2010 American Institute of Physics.
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| 2. |
- Echt, O., et al.
(författare)
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Vibrational Energy Dependence of the Triplet Lifetime in Isolated, Photo-Excited C60
- 2004
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Ingår i: J. Phys. Chem. A. - : American Chemical Society (ACS). - 1089-5639. ; 108, s. 6944-6952
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Tidskriftsartikel (refereegranskat)abstract
- We have measured the lifetime of the lowest triplet state T1 in free C60 by a pump-probe experiment using lasers with nanosecond pulse durations. At low pump fluence the population of T1 decays with a distinct, narrow distribution of lifetimes. depends on the pump wavelength ( = 532, 355, or 266 nm) as well as the temperature of the source from which C60 is vaporized (420 T 510 C); it ranges from 2 s to 0.3 s. At high pump fluence an additional lifetime as short as 40 ns is observed. A consistent correlation of all observed lifetimes with the experimental parameters is found if is assigned to an ensemble of C60 (T1) that has absorbed either one or two pump photons with the excess energy being randomized over all vibrational modes. Thus, = 2 s corresponds to a vibrational energy Evib = Etotal - Etriplet = 4.6 eV (one-photon absorption at = 532 nm, T = 420 C) while = 40 ns corresponds to Evib = 9.6 eV (two-photon absorption at 355 nm, 480 C). This result strongly suggests that delayed electrons that are emitted from highly excited C60 (Evib 10 eV) on the time scale of 10 ns to 1 ms are not affected by long-lived electronically excited states. The frequently questioned description of delayed electron emission from photoexcited C60 as thermionic emission is, therefore, warranted.
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| 3. |
- Hansen, Klavs, 1958, et al.
(författare)
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The frequency factor in statistical fullerene decay
- 2006
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Ingår i: Int. J. Mass Spec.. - : Elsevier BV. - 1387-3806. ; 252, s. 79-95
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Tidskriftsartikel (refereegranskat)abstract
- Experiments on fullerene decay are reviewed and the frequency factor for C2 emission is extracted. The value is also calculated theoretically. Inclusion of a number of previously disregarded degrees of freedom for the products increase the theoretical estimate for C60 to above 1020 s−1, in good agreement with experimental results.
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| 4. |
- Kadhane, U., et al.
(författare)
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Near-infrared photoabsorption by C-60 dianions in a storage ring
- 2009
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Ingår i: Journal of Chemical Physics. - : AIP Publishing. - 0021-9606 .- 1089-7690. ; 131:1
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Tidskriftsartikel (refereegranskat)abstract
- We present a detailed study of the electronic structure and the stability of C-60 dianions in the gas phase. Monoanions were extracted from a plasma source and converted to dianions by electron transfer in a Na vapor cell. The dianions were then stored in an electrostatic ring, and their near-infrared absorption spectrum was measured by observation of laser induced electron detachment. From the time dependence of the detachment after photon absorption, we conclude that the reaction has contributions from both direct electron tunneling to the continuum and vibrationally assisted tunneling after internal conversion. This implies that the height of the Coulomb barrier confining the attached electrons is at least similar to 1.5 eV. For C-60(2-) ions in solution electron spin resonance measurements have indicated a singlet ground state, and from the similarity of the absorption spectra we conclude that also the ground state of isolated C-60(2-) ions is singlet. The observed spectrum corresponds to an electronic transition from a t(1u) lowest unoccupied molecular orbital (LUMO) of C-60 to the t(1g) LUMO+1 level. The electronic levels of the dianion are split due to Jahn-Teller coupling to quadrupole deformations of the molecule, and a main absorption band at 10723 cm(-1) corresponds to a transition between the Jahn-Teller ground states. Also transitions from pseudorotational states with 200 cm(-1) and (probably) 420 cm(-1) excitation are observed. We argue that a very broad absorption band from about 11 500 cm(-1) to 13 500 cm(-1) consists of transitions to so-called cone states, which are Jahn-Teller states on a higher potential-energy surface, stabilized by a pseudorotational angular momentum barrier. A previously observed, high-lying absorption band for C-60(-) may also be a transition to a cone state.
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| 5. |
- Mauracher, Andreas, et al.
(författare)
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Metastable anions of dinitrobenzene : Resonances for electron attachment and kinetic energy release
- 2010
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Ingår i: Journal of Chemical Physics. - : AIP Publishing. - 0021-9606 .- 1089-7690. ; 133:24, s. 244302-
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Tidskriftsartikel (refereegranskat)abstract
- Attachment of free, low-energy electrons to dinitrobenzene (DNB) in the gas phase leads to DNB− as well as several fragment anions. DNB−, (DNB-H)−, (DNB-NO)−, (DNB-2NO)−, and (DNB-NO2)− are found to undergo metastable (unimolecular) dissociation. A rich pattern of resonances in the yield of these metastable reactions versus electron energy is observed; some resonances are highly isomer-specific. Most metastable reactions are accompanied by large average kinetic energy releases (KER) that range from 0.5 to 1.32 eV, typical of complex rearrangement reactions, but (1,3-DNB-H)− features a resonance with a KER of only 0.06 eV for loss of NO. (1,3-DNB-NO)− offers a rare example of a sequential metastable reaction, namely, loss of NO followed by loss of CO to yield C5H4O− with a large KER of 1.32 eV. The G4(MP2) method is applied to compute adiabatic electron affinities and reaction energies for several of the observed metastable channels
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| 6. |
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| 7. |
- Tomita, S., et al.
(författare)
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Dynamic Jahn-Teller effects in isolated C60- studied by near-infrared spectroscopy in a storage ring
- 2005
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Ingår i: Phys. Rev. Lett.. - 0031-9007. ; 94
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Tidskriftsartikel (refereegranskat)abstract
- We have measured the near-infrared absorption spectrum for isolated C ions at room temperature. Two bands, at 9145 cm-1 and 10460 cm-1, have been identified in addition to the main absorption band at 9382 cm-1, seen also at low temperature in a matrix. An interpretation based on the theory of dynamic Jahn-Teller effects is proposed.
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| 8. |
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| 9. |
- Tomita, S., et al.
(författare)
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Lifetimes of C602- and C702- dianions in a storage ring
- 2006
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Ingår i: J. Chem. Phys.. - : AIP Publishing. - 0021-9606. ; 124
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Tidskriftsartikel (refereegranskat)abstract
- C602- and C702- 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 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 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.
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