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ABSTRACT: In the present study, dissociative electron attachment (DEA) measurements with gas phase HMX, octahydro-1,3,5,7-tetranitro-1,3,5,7-tetrazocine, C4H8N8O8, have been performed by means of a crossed electron-molecular beam experiment. The most intense signals are observed at 46 and 176 u and assigned to NO2 - and C3H6N5O4 -, respectively. Anion efficiency curves for 15 negatively charged fragments have been measured in the electron energy region from about 0-20 eV with an energy resolution of ~0.7 eV. Product anions are observed mainly in the low energy region, near 0 eV, arising from surprisingly complex reactions associated with multiple bond cleavages and structural and electronic rearrangement. The remarkable instability of HMX towards electron attachment with virtually zero kinetic energy reflects the highly explosive nature of this compound. Substantially different intensity ratios of resonances for common fragment anions allow distinguishing the nitroamines HMX and royal demolition explosive molecule (RDX) in negative ion mass spectrometry based on free electron capture.
Journal of the American Society for Mass Spectrometry 03/2013; · 4.00 Impact Factor
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ABSTRACT: The electron-driven ionization of helium droplets doped with pure methanol and ethanol clusters has been investigated for the first time using high resolution mass spectrometry. Large clusters are readily accessible by this route, with up to 100 alcohol molecules seen in the present study. The mass spectra for the doped helium droplets show many similarities with previous gas phase mass spectrometric studies of methanol and ethanol clusters. Thus the dominant ion products, at least for small clusters, are the protonated species H(+)(CH(3)OH)(n) and H(+)(C(2)H(5)OH)(n). Likewise intra-cluster reaction is observed to produce H(+)(H(2)O)(CH(3)OH)(n) and H(+)(H(2)O)(C(2)H(5)OH)(n) ions. However, in helium droplets the observation of consecutive intra-cluster reactions is seen with product molecules containing up to five water molecules. The evidence points towards the proton locating on H(2)O to form H(3)O(+), rather than the alcohol, despite the higher proton affinity of the latter. The behaviour of the H(+)(H(2)O)(m)(ROH)(n) ion signals as a function of cluster size is consistent with the most stable cluster structures arising from a central H(3)O(+) ion surrounded by two or more complete five-membered rings with the constituents held in place by hydrogen bonds.
Physical Chemistry Chemical Physics 02/2013; · 3.57 Impact Factor
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ABSTRACT: Bundles of single-walled nanotubes are promising candidates for storage of hydrogen, methane, and other hydrogen-rich molecules, but experiments are hindered by nonuniformity of the tubes. We overcome the problem by investigating methane adsorption on aggregates of fullerenes containing up to six C(60); the systems feature adsorption sites similar to those of nanotube bundles. Four different types of adsorption sites are distinguished, namely, registered sites above the carbon hexagons and pentagons, groove sites between adjacent fullerenes, dimple sites between three adjacent fullerenes, and exterior sites. The nature and adsorption energies of the sites in C(60) aggregates are determined by density functional theory and molecular dynamics (MD) simulations. Excellent agreement between experiment and theory is obtained for the adsorption capacity in these sites.
Journal of Physical Chemistry Letters 09/2012; 3(18):2598-2603. · 6.21 Impact Factor
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ABSTRACT: Energetic and geometric aspects of the permeation of low-Z atoms through
graphene sheets are investigated. Energy barriers and deformations are
calculated via density functional theory for the permeation of H, He, Li and Be
atoms at several surface sites and at a hollow site for atoms B, C, O and Ne
atoms. Graphene is modeled by large planar polycyclic aromatic hydrocarbons and
the convergence of both energy barriers and deformation curves with increasing
size of these hydrocarbons is investigated. Effective energy curves are
summarized for the atoms under consideration in three different interaction
regimes realized different geometrical constraints. In addition to the bare
graphene model, the interaction between low-Z atoms and 100% hydrogenated
coronene as a model for graphane is also investigated. The adiabatic barriers
range from about 5 eV (1 eV = 1.602 x 10-19 J) for H to about 20 eV for Ne.
Facilitation of the permeation by temporary chemical bonding is observed for O
and C and for B and Be when interacting with hydrogenated coronene. The results
are in good agreement with existing experimental and theoretical work and
reflect the essential physics of the dynamics of H bombardment of graphite
(0001) surfaces. Implications for modeling chemical sputtering of graphite in a
mixed-material scenario as it will be the case in the next step fusion
experiment ITER and potential building are discussed.
01/2012;
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Elahe Alizadeh,
David Gschliesser,
Peter Bartl,
Michaela Hager,
Achim Edtbauer,
Violaine Vizcaino, Andreas Mauracher,
Michael Probst,
Tilmann D Märk,
Sylwia Ptasińska,
Nigel J Mason,
Stephan Denifl,
Paul Scheier
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ABSTRACT: Dissociative electron attachment to dialanine and alanine anhydride has been studied in the gas phase utilizing a double focusing two sector field mass spectrometer. We show that low-energy electrons (i.e., electrons with kinetic energies from near zero up to 13 eV) attach to these molecules and subsequently dissociate to form a number of anionic fragments. Anion efficiency curves are recorded for the most abundant anions by measuring the ion yield as a function of the incident electron energy. The present experiments show that as for single amino acids (M), e.g., glycine, alanine, valine, and proline, the dehydrogenated closed shell anion (M-H)(-) is the most dominant reaction product. The interpretation of the experiments is aided by quantum chemical calculations based on density functional theory, by which the electrostatic potential and molecular orbitals are calculated and the initial electron attachment process prior to dissociation is investigated.
The Journal of chemical physics 02/2011; 134(5):054305. · 3.09 Impact Factor
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Harald Schöbel,
Peter Bartl,
Christian Leidlmair,
Matthias Daxner,
Samuel Zöttl,
Stephan Denifl,
Tilmann D Märk,
Paul Scheier,
Daniel Spångberg, Andreas Mauracher,
Diethard K Bohme
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ABSTRACT: We report the observation of the ejection of electrons caused by collisions of excited atoms with ions, rather than neutrals, leading to the production of doubly charged ions. Doping superfluid He droplets with methyl iodide and exposing them to electrons enhances the formation of doubly charged iodine atoms at the threshold for the production of two metastable He atoms. These observations point toward a novel ionization process where doubly charged ions are produced by sequential Penning ionization. In some cases, depending on the neutral target, the process also leads to a subsequent Coulomb explosion of the dopant.
Physical Review Letters 12/2010; 105(24):243402. · 7.37 Impact Factor
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ABSTRACT: Electron attachment to the explosive trinitrotoluene (TNT) embedded in Helium droplets (TNT@He) generates the non-decomposed complexes (TNT)(n)(-), but no fragment ions in the entire energy range 0-12 eV. This strongly contrasts the behavior of single TNT molecules in the gas phase at ambient temperatures, where electron capture leads to a variety of different fragmentation products via different dissociative electron attachment (DEA) reactions. Single TNT molecules decompose by attachment of an electron at virtually no extra energy reflecting the explosive nature of the compound. The complete freezing of dissociation intermediates in TNT embedded in the droplet is explained by the particular mechanisms of DEA in nitrobenzenes, which is characterized by complex rearrangement processes in the transient negative ion (TNI) prior to decomposition. These mechanisms provide the condition for effective energy withdrawal from the TNI into the dissipative environment thereby completely suppressing its decomposition.
Physical Chemistry Chemical Physics 10/2009; 11(37):8240-3. · 3.57 Impact Factor
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ABSTRACT: Helium nanodroplets are doped with SF(6), C(4)F(8), CCl(4), C(6)H(5)Br, CH(3)I, and I(2). Upon interaction with free electrons a variety of positively and negatively charged cluster ions X(+/-)He(n) are observed where X(+/-) = F(+/-), Cl(+/-), Br(+/-), I(+), I(2) (+), or CH(3)I(+). The yield of these ions versus cluster size n drops at characteristic sizes n(s) that range from n(s) = 10.2+/-0.6 for F(+) to n(s) = 22.2+/-0.2 for Br(-). n(s) values for halide anions are about 70% larger than for the corresponding cations. The steps in the ion yield suggest closure of the first solvation shell. We propose a simple classical model to estimate ionic radii from n(s). Assuming the helium density in the first solvation shell equals the helium bulk density one finds that radii of halide anions in helium are nearly twice as large as in alkali halide crystals, indicating the formation of an anion bubble due to the repulsive forces that derive from the exchange interaction. In spite of the simplicity of our model, anion radii derived from it agree within approximately 10% with values derived from the mobility of halide anions in superfluid bulk helium, and with values computed by quantum Monte Carlo methods for X(-)He(n) cluster anions.
Chemistry 07/2009; 15(29):7101-8. · 5.93 Impact Factor
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Chemistry 06/2009; 15(29):7101 - 7108. · 5.93 Impact Factor
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Stephan Denifl,
Fabio Zappa,
Ingo Mähr,
Filipe Ferreira da Silva,
Abid Aleem, Andreas Mauracher,
Michael Probst,
Jan Urban,
Pavel Mach,
Arntraud Bacher,
Olof Echt,
Tilmann D Märk,
Paul Scheier
Angewandte Chemie International Edition 01/2009; 48(47):8940-3. · 13.45 Impact Factor
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ChemPhysChem 08/2008; 9(10):1387-9. · 3.41 Impact Factor
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ABSTRACT: The inelastic electron interaction (ionization/attachment) with chloroform embedded in helium droplets has been studied utilizing a two-sector field mass spectrometer. Positive mass spectra have been recorded at the electron energy of 70 eV and are compared with previous results in the gas phase and with other systems embedded in helium droplets. Moreover, the negative ion mass spectrum has been recorded at the electron energy of 1.5 eV. Both negative and positive mass spectra show that chloroform clusters are easily formed by embedding single molecules in the helium droplets. Moreover, for anions appearing in the mass spectrum, the ion yield has been determined as function of the electron energy. While no parent anion of chloroform can be observed in the gas phase, the present cluster environment allows the stabilization of the transient negative ion. The influence of the helium droplet upon the ionization or attachment process of the embedded chloroform is discussed.
Journal of the American Chemical Society 05/2008; 130(15):5065-71. · 9.91 Impact Factor
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Fabio Zappa,
Manuel Beikircher, Andreas Mauracher,
Stephan Denifl,
Michael Probst,
Natcha Injan,
Jumras Limtrakul,
Arntraud Bacher,
Olof Echt,
Tilmann D Märk,
Paul Scheier,
Thomas A Field,
Karola Graupner
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ABSTRACT: A comprehensive analysis of metastable dissociation of 2,4-dinitrotoluene (DNT) parent anions formed by attachment of electrons of controlled energy is presented. We characterize the energy dependence and kinetic energy release of the reaction which competes with autodetachment. A surprising finding is a highly exothermic metastable reaction triggered by the attachment of thermal electrons which we relate to the well-known electrostatic ignition hazards of DNT and other explosives. Quantum chemical calculations are performed for dinitrobenzene in order to elucidate the process of NO abstraction.
ChemPhysChem 04/2008; 9(4):607-11. · 3.41 Impact Factor
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Philipp Sulzer, Andreas Mauracher,
Stephan Denifl,
Fabio Zappa,
Sylwia Ptasinska,
Manuel Beikircher,
Arntraud Bacher,
Nina Wendt,
Abid Aleem,
Flaminia Rondino,
Stefan Matejcik,
Michael Probst,
Tilmann D Märk,
Paul Scheier
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ABSTRACT: Free electron attachment to the three different isomers of mononitrotoluene molecules in the gas phase is studied using a crossed electron-molecule beams technique. In contrast to previous studies for a large number of negative ions, the presently measured relative cross section curves are recorded with an electron energy resolution of better than 100 meV. For several product anions including the nitro anion NO(2)-, remarkable differences for the three isomers are observed. In almost all fragment anion efficiency curves, the 2-nitrotoluene exhibits pronounced differences from the two other isomers. In contrast, 3- and 4-nitrotoluene disagree only slightly in a few fragment anions from each other.
Analytical Chemistry 10/2007; 79(17):6585-91. · 5.86 Impact Factor
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Angewandte Chemie International Edition 02/2007; 46(42):8057-9. · 13.45 Impact Factor
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Michael Probst,
Natcha Injan,
Stephan Denifl,
Fabio Zappa,
Ingo Mahr,
Manuel Beikircher,
Sylwia Ptasinska,
Jumras Limtrakul,
Tilmann D. Mark, Andreas Mauracher,
Paul Scheier
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ABSTRACT: Electron-molecule reactions and interactions are of increasing importance in science and technology. This already includes wide areas of environmental physics, technological plasmas, astrophysics, electron-driven chemistry, and fusion research. In this work we give a short overview of the topic and turn to two specific examples that show how synergies between experiments and calculations can lead to progress in the field. The first example concerns fragmentation of an ionized species. It has been found that hydrogen atoms are ejected from a specific position if an electron interacts with an adenine molecule. This process is prototypical for many electron-driven reactions and its understanding—which has not yet been fully achieved—is very important for a variety of processes in fundamental physics, biophysics, chemistry, and technology. The second example concerns mixtures of adenine and thymine and their ionization. Binding energies and structures of neutral and ionic nucleic acid dimers AA, AT, and TT (A = adenine, T = thymine), which are formed in such mixtures, have been calculated. The results predict which of the species will occur preferably in a comparable experimental situation and show for which cases the geometries of charged dimers differ from the neutral ones.
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ABSTRACT: Free electron attachment to the three isomers of di-nitrobenzene (DNB) by means of a crossed electron molecular beam experiment with mass spectrometric detection of the anions is studied. In all three isomers, we detect long lived (metastable) non-dissociated parent anions which are formed at energies near 0 eV. In addition, all three compounds exhibit rich fragmentation patterns due to dissociative electron attachment (DEA). The resonance profiles of particular fragment anions in the energy range 0–10 eV differ substantially between the different isomers which can be used for an unambiguous identification of the individual isomer. Many of the DEA products observed at low energy arise from surprisingly complex reactions associated with multiple bond cleavages and structural and electronic rearrangement.
International Journal of Mass Spectrometry.
