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Träfflista för sökning "WFRF:(Tielens Alexander G. G. M.) "

Sökning: WFRF:(Tielens Alexander G. G. M.)

  • Resultat 1-10 av 11
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1.
  • Seitz, Fabian, et al. (författare)
  • Ions colliding with clusters of fullerenes-Decay pathways and covalent bond formations
  • 2013
  • Ingår i: Journal of Chemical Physics. - 0021-9606 .- 1089-7690. ; 139:3
  • Tidskriftsartikel (refereegranskat)abstract
    • We report experimental results for the ionization and fragmentation of weakly bound van der Waals clusters of n C-60 molecules following collisions with Ar2+, He2+, and Xe20+ at laboratory kinetic energies of 13 keV, 22.5 keV, and 300 keV, respectively. Intact singly charged C-60 monomers are the dominant reaction products in all three cases and this is accounted for by means of Monte Carlo calculations of energy transfer processes and a simple Arrhenius-type [C-60](n)(+) -> C-60(+) + (n - 1)C-60 evaporation model. Excitation energies in the range of only similar to 0.7 eV per C-60 molecule in a [C-60](13)(+) cluster are sufficient for complete evaporation and such low energies correspond to ion trajectories far outside the clusters. Still we observe singly and even doubly charged intact cluster ions which stem from even more distant collisions. For penetrating collisions the clusters become multiply charged and some of the individual molecules may be promptly fragmented in direct knock-out processes leading to efficient formations of new covalent systems. For Ar2+ and He2+ collisions, we observe very efficient C-119(+) and C-118(+) formation and molecular dynamics simulations suggest that they are covalent dumb-bell systems due to bonding between C-59(+) or C-58(+) and C-60 during cluster fragmentation. In the Ar2+ case, it is possible to form even smaller C-120-2m(+) molecules (m = 2-7), while no molecular fusion reactions are observed for the present Xe20+ collisions.
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2.
  • Zettergren, Henning, et al. (författare)
  • Bond formation in C-59(+)-C-60 collisions
  • 2014
  • Ingår i: XXVIII International Conference on Photonic, Electronic and Atomic Collisions (ICPEAC 2013). - : Institute of Physics (IOP).
  • Konferensbidrag (refereegranskat)abstract
    • In this work, we show that keV-ions are able to remove single carbon atoms from individual fullerenes in clusters of C-60 molecules. This very efficiently leads to the formation of exotic q dumbbell molecules through secondary C-59(+) - C-60 collisions within the fragmenting cluster. Such molecular fusion processes are inherently different from those induced by photons where only products with even numbers of carbon atoms are observed. Thus, ion collisions ignite unique and hitherto overlooked secondary reactions in small aggregates of matter. This relates to the question on how complex molecules may form in e.g. space.
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3.
  • Zettergren, Henning, et al. (författare)
  • Formation of dumb-bell C118 and C119 inside clusters of C60 -molecules
  • Tidskriftsartikel (refereegranskat)abstract
    • We report highly selective covalent bond-modifications in collisions between keV alpha particles and van der Waals clusters of C60-fullerenes. Surprisingly, C119+ and C118+ are the dominant molecular fusion products. We use Molecular Dynamics simulations to show that C59+ and C58+ ions - effectively produced in prompt knock-out processes with He2+ - react rapidly with C60 to form dumb-bell C119+ and C118+ . Ion impact on molecular clusters in general is expected to lead to efficient secondary reactions of interest for astrophysics. These reactions are different from those induced by photons.
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4.
  • Zettergren, Henning, et al. (författare)
  • Formations of Dumbbell C-118 and C-119 inside Clusters of C-60 Molecules by Collision with alpha Particles
  • 2013
  • Ingår i: Physical Review Letters. - 0031-9007 .- 1079-7114. ; 110:18
  • Tidskriftsartikel (refereegranskat)abstract
    • We report highly selective covalent bond modifications in collisions between keV alpha particles and van der Waals clusters of C-60 fullerenes. Surprisingly, C-119(+) and C-118(+) are the dominant molecular fusion products. We use molecular dynamics simulations to show that C-59(+) and C-58(+) ions-effectively produced in prompt knockout processes with He2+-react rapidly with C-60 to form dumbbell C-119(+) and C-118(+). Ion impact on molecular clusters in general is expected to lead to efficient secondary reactions of interest for astrophysics. These reactions are different from those induced by photons.
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5.
  • Chen, Tao, et al. (författare)
  • Anharmonicity and the infrared emission spectrum of highly excited polycyclic aromatic hydrocarbons
  • 2018
  • Ingår i: Astronomy and Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 618
  • Tidskriftsartikel (refereegranskat)abstract
    • Aims. Infrared (IR) spectroscopy is a powerful tool to study molecules in space. A key issue in such analyses is understanding the effect that temperature and anharmonicity have on different vibrational bands, and thus interpreting the IR spectra for molecules under various conditions. Methods. We combined second order vibrational perturbation theory and the Wang-Landau random walk technique to produce accurate IR spectra of highly excited polycyclic aromatic hydrocarbons. We fully incorporated anharmonic effects, such as resonances, overtones, combination bands, and temperature effects. Results. The results are validated against experimental results for the pyrene molecule (C16H10). In terms of positions, widths, and relative intensities of the vibrational bands, our calculated spectra are in excellent agreement with gas-phase experimental data.
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6.
  • Chen, Tao, et al. (författare)
  • Formation of H2 from internally heated polycyclic aromatic hydrocarbons : Excitation energy dependence
  • 2015
  • Ingår i: Journal of Chemical Physics. - 0021-9606 .- 1089-7690. ; 142:14
  • Tidskriftsartikel (refereegranskat)abstract
    • We have investigated the effectiveness of molecular hydrogen (H-2) formation from Polycyclic Aromatic Hydrocarbons (PAHs) which are internally heated by collisions with keV ions. The present and earlier experimental results are analyzed in view of molecular structure calculations and a simple collision model. We estimate that H-2 formation becomes important for internal PAH temperatures exceeding about 2200 K, regardless of the PAH size and the excitation agent. This suggests that keV ions may effectively induce such reactions, while they are unlikely due to, e.g., absorption of single photons with energies below the Lyman limit. The present analysis also suggests that H-2 emission is correlated with multi-fragmentation processes, which means that the [PAH-2H](+) peak intensities in the mass spectra may not be used for estimating H-2-formation rates.
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7.
  • Chen, Tao, et al. (författare)
  • Temperature effects on the infrared spectrum of molecules in planetary nebulae
  • 2017
  • Ingår i: Planetary nebulae. - : CAMBRIDGE UNIV PRESS. - 9781107169913 ; , s. 357-358
  • Konferensbidrag (refereegranskat)abstract
    • In this work, we utilize a method based on Wang-Landau Monte Carlo sampling for studying the temperature effects of astrophysically relavant molecules. Anharmonic effects, e.g., resonances, overtones, and combination bands, are fully incoportated in this method. The calculated infrared (IR) spectra are consistent with the experimental data measured by National Institute of Standards and Technology (NIST).
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8.
  • Emig, Kimberly L., et al. (författare)
  • Super Star Clusters in the Central Starburst of NGC 4945
  • 2020
  • Ingår i: Astrophysical Journal. - 1538-4357 .- 0004-637X. ; 903:1
  • Tidskriftsartikel (refereegranskat)abstract
    • The nearby (3.8Mpc) galaxy NGC 4945 hosts a nuclear starburst and Seyfert type 2 active galactic nucleus (AGN). We use the Atacama Large Millimeter/submillimeter Array (ALMA) to image the 93 GHz (3.2 mm) free-free continuum and hydrogen recombination line emission (H40 alpha and H42 alpha) at 2.2 pc (0 12) resolution. Our observations reveal 27 bright, compact sources with FWHM sizes of 1.4-4.0 pc, which we identify as candidate super star clusters. Recombination line emission, tracing the ionizing photon rate of the candidate clusters, is detected in 15 sources, six of which have a significant synchrotron component to the 93 GHz continuum. Adopting an age of similar to 5Myr, the stellar masses implied by the ionizing photon luminosities are log(10) (M*/M-circle dot) approximate to 4.7-6.1. We fit a slope to the cluster mass distribution and find beta = -1.8 +/-.0.4. The gas masses associated with these clusters, derived from the dust continuum at 350 GHz, are typically an order of magnitude lower than the stellar mass. These candidate clusters appear to have already converted a large fraction of their dense natal material into stars and, given their small freefall times of similar to 0.05 Myr, are surviving an early volatile phase. We identify a pointlike source in 93 GHz continuum emission that is presumed to be the AGN. We do not detect recombination line emission from the AGN and place an upper limit on the ionizing photons that leak into the starburst region of Q(0).<.10(52) s(-1).
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9.
  • Mackie, Cameron J., et al. (författare)
  • Fully anharmonic infrared cascade spectra of polycyclic aromatic hydrocarbons
  • 2018
  • Ingår i: Journal of Chemical Physics. - : American Institute of Physics (AIP). - 0021-9606 .- 1089-7690. ; 149:13
  • Tidskriftsartikel (refereegranskat)abstract
    • The infrared (IR) emission of polycyclic aromatic hydrocarbons (PAHs) permeates our universe; astronomers have detected the IR signatures of PAHs around many interstellar objects. The IR emission of interstellar PAHs differs from their emission as seen under conditions on Earth as they emit through a collisionless cascade down through their excited vibrational states from high internal energies. The difficulty in reproducing interstellar conditions in the laboratory results in a reliance on theoretical techniques. However, the size and complexity of PAHs require careful consideration when producing the theoretical spectra. In this work, we outline the theoretical methods necessary to lead to fully theoretical IR cascade spectra of PAHs including: an anharmonic second order vibrational perturbation theory treatment, the inclusion of Fermi resonances through polyads, and the calculation of anharmonic temperature band shifts and broadenings (including resonances) through a Wang-Landau approach. We also suggest a simplified scheme to calculate vibrational emission spectra that retain the essential characteristics of the full IR cascade treatment and can directly transform low temperature absorption spectra in IR cascade spectra. Additionally we show that past astronomical models were in error in assuming a 15 cm(-1) correction was needed to account for anharmonic emission effects.
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10.
  • Zhang, Weiwei, et al. (författare)
  • Laboratory Photochemistry of Covalently Bonded Fluorene Clusters : Observation of an Interesting PAH Bowl-forming Mechanism
  • 2019
  • Ingår i: Astrophysical Journal. - : IOP PUBLISHING LTD. - 0004-637X .- 1538-4357. ; 872:1
  • Tidskriftsartikel (refereegranskat)abstract
    • The fullerene C-60, one of the largest molecules identified in the interstellar medium (ISM), has been proposed to form top-down through the photochemical processing of large (more than 60 C atoms) polycyclic aromatic hydrocarbon (PAH) molecules. In this article, we focus on the opposite process, investigating the possibility that fullerenes form from small PAHs, in which bowl-forming plays a central role. We combine laboratory experiments and quantum chemical calculations to study the formation of larger PAHs from charged fluorene clusters. The experiments show that with visible laser irradiation, the fluorene dimer cation-[C13H9-C13H9](+)-and the fluorene trimer cation-[C13H9-C13H8-C13H9](+)-undergo photodehydrogenation and photoisomerization, resulting in bowl-structured aromatic cluster ions, C26H12+ and C39H20+, respectively. To study the details of this chemical process, we employ quantum chemistry that allows us to determine the structures of the newly formed cluster ions, to calculate the dissociation energies for hydrogen loss, and to derive the underlying reaction pathways. These results demonstrate that smaller PAH clusters (with less than 60 C atoms) can convert to larger bowled geometries that might act as building blocks for fullerenes, because the bowl-forming mechanism greatly facilitates the conversion from dehydrogenated PAHs to cages. Moreover, the bowl-forming induces a permanent dipole moment that-in principle-allows one to search for such species using radio astronomy.
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  • Resultat 1-10 av 11
  • [1]2Nästa

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