| 1. |
- Stockett, Mark H., et al.
(författare)
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Fragmentation of anthracene C14H10, acridine C13H9N and phenazine C12H8N2 ions in collisions with atoms
- 2014
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Ingår i: Physical Chemistry, Chemical Physics - PCCP. - : Royal Society of Chemistry (RSC). - 1463-9076 .- 1463-9084. ; 16:40, s. 21980-21987
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Tidskriftsartikel (refereegranskat)abstract
- We report experimental total, absolute, fragmentation cross sections for anthracene C14H10, acridine C13H9N, and phenazine C12H8N2 ions colliding with He at center-of-mass energies close to 100 eV. In addition, we report results for the same ions colliding with Ne, Ar, and Xe at higher energies. The total fragmentation cross sections for these three ions are the same within error bars for a given target. The measured fragment mass distributions reveal significant contributions from both delayed (>> 10(-12) s) statistical fragmentation processes as well as non-statistical, prompt (similar to 10(-15) s), single atom knockout processes. The latter dominate and are often followed by secondary statistical fragmentation. Classical Molecular Dynamics (MD) simulations yield separate cross sections for prompt and delayed fragmentation which are consistent with the experimental results. The intensity of the single C/N-loss peak, the signature of non-statistical fragmentation, decreases with the number of N atoms in the parent ion. The fragment intensity distributions for losses of more than one C or N atom are rather similar for C14H10 and C13H9N but differ strongly for C12H8N2 where weak C-N bonds often remain in the fragments after the first fragmentation step. This greatly increases their probability to fragment further. Distributions of internal energy remaining in the fragments after knockout are obtained from the MD simulations.
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| 2. |
- Fischer, D., et al.
(författare)
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Importance of Thomas single-electron transfer in fast p-He collisions
- 2010
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Ingår i: Physical Review A. Atomic, Molecular, and Optical Physics. - 1050-2947 .- 1094-1622. ; 81:1, s. 12714-
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Tidskriftsartikel (refereegranskat)abstract
- We report experimental angular differential cross sections for nonradiative single-electron capture in p-He collisions (p + He -> H + He+) with a separate peak at the 0.47 mrad Thomas scattering angle for energies in the 1.3-12.5 MeV range. We find that the intensity of this peak scales with the projectile velocity as v(P)(-11). This constitutes the first experimental test of the prediction from 1927 by L. H. Thomas [Proc. R. Soc. 114, 561 (1927)]. At our highest energy, the peak at the Thomas angle contributes with 13.5% to the total integrated nonradiative single-electron capture cross section.
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| 3. |
- Gudmundsson, Magnus, et al.
(författare)
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Angular scattering in fast ion-atom electron transfer collisions : projectile wave diffraction and Thomas mechanisms
- 2010
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Ingår i: Journal of Physics B. - : IOP Publishing. - 0953-4075 .- 1361-6455. ; 43:18, s. 185209-
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Tidskriftsartikel (refereegranskat)abstract
- We report experimental angular differential cross sections for double-electron capture in He2+ + He collisions and single-electron capture in H+ + He collisions for the 1.3-12.5 MeV kinetic energy range. In all cases, the total cross sections are dominated by forward scattering peaks in d sigma/d Omega. The shapes and widths (but not the magnitudes) of these peaks are very similar for all energies and for capture of one or two electrons corresponding also to our measured linear increases in the transverse momentum transfers with increasing projectile velocities. These observations may be ascribed to diffraction limitations which are connected to electron transfer probabilities P(b) which are significant in limited regions of b only. For the H+ + He single-electron capture we observe two additional maxima in the angular differential cross sections. We conclude that while the secondary maxima at similar to 0.5 mrad probably have large contributions from the Thomas proton-electron-nucleus scattering mechanism, the third maxima at similar to 0.75 mrad are most likely mainly due to projectile de Broglie wave diffraction.
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| 4. |
- Hvelplund, P., et al.
(författare)
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Stability and Structure of Protonated Clusters of Ammonia and Water, H+(NH3)(m) (H2O)(n)
- 2010
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Ingår i: Journal of Physical Chemistry A. - : American Chemical Society (ACS). - 1089-5639 .- 1520-5215. ; 114:27, s. 7301-7310
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Tidskriftsartikel (refereegranskat)abstract
- Mass spectrometric experiments show that protonated mixed ammonia/water clusters predominant exist in three forms namely H+(NH3)(4)(H2O)(n), H+(NH3)(5)(H2O)(n), and H+(NH3)(6)(H2O)(n) (n = 1-25). For the first two series the collisional activation mass spectra are dominated by loss of water, whereas ions of the latter series preferably lose ammonia. The quantitative characteristics of these observations are reproduced by quantum chemical calculations that also provide insight into the geometrical structures of the clusters. Although the experiments and the calculations agree that clusters with five ammonia are thermodynamically preferred, this does not indicate a rigid tetrahedral structure with one central ammonium covered with an inner solvation shell of four ammonia molecules, with water outside, Instead, water and ammonia have comparable affinities to the binding sites of the first shell, with a preference for ammonia for the first two sites, and water for the last two. The "leftover" ammonia molecules bind equally strong as water molecules to sites in the second shell due to synergistic hydrogen binding. Finally, it is discussed whether the observation of enhanced stability of the H+(NH3)(5)(H2O)(20) in terms of magic numbers and associated geometries may be related to a tetrahedral ammonium core encapsulated in a dodecahedral (H2O)(20) structure, typically found in clathrates.
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| 5. |
- 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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| 6. |
- Sundén, Erika, 1970, et al.
(författare)
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Stabilities of protonated water-ammonia clusters
- 2018
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Ingår i: Journal of Chemical Physics. - : AIP Publishing. - 0021-9606 .- 1089-7690. ; 148:18
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Tidskriftsartikel (refereegranskat)abstract
- Branching ratios of water and ammonia evaporation have been measured for spontaneous evaporation from protonated mixed clusters H+(H2On(NH3)(m) in the size range 0 <= n <= 11 and 0 <= m <= 7. Mixed clusters evaporate water except for clusters containing six or more ammonia molecules, indicating the formation of a stable core of one ammonium ion surrounded by four ammonia molecules and a second shell consisting predominantly of water. We relate evaporative branching ratios to free energy differences between the products of competing channels and determine the free energy differences for clusters with up to seven ammonia molecules. Clusters containing up to five ammonia molecules show a very strong scaling of these free energy differences. Published by AIP Publishing.
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| 7. |
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