Stephan Denifl

University of Innsbruck, Innsbruck, Tyrol, Austria

Are you Stephan Denifl?

Claim your profile

Publications (171)527.75 Total impact


  • No preview · Article · Sep 2015 · Journal of Physics Conference Series

  • No preview · Article · Sep 2015 · Journal of Physics Conference Series

  • No preview · Article · Sep 2015 · Journal of Physics Conference Series
  • [Show abstract] [Hide abstract]
    ABSTRACT: We study the dissociative ionization of Ar dimer (Ar2) by electron impact with the multiple particles coincidence (electrons and ions) method using a reaction microscope. From the measured kinetic energies of final state electrons and ions, we identify two decay channels which can contribute to the double peak structure in the observed kinetic energy release (KER) spectrum of argon Ar2 dissociating into Ar+- Ar+.
    No preview · Article · Sep 2015 · Journal of Physics Conference Series
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: We describe the construction of an apparatus for the production of a molecular jet of microhydrated biomolecules. Our design uses a water reservoir producing water vapour, which then passes through a separate reservoir containing a vapour of a sublimated biomolecule. The mixture coexpands into a molecular beam apparatus through a conical nozzle. Mass spectra showing water-adenin and water-uracil complexes are shown as typical examples. Suitable expansion conditions are reached without the use of an inert carrier gas.
    Full-text · Article · Jul 2015 · The Review of scientific instruments
  • [Show abstract] [Hide abstract]
    ABSTRACT: The interaction of low-energy electrons with biomolecules plays an important role in the radiation-induced alteration of biological tissue at the molecular level. At electron energies below 15 eV, dissociative electron attachment is one of the most important processes in terms of the chemical transformation of molecules. So far, a common approach to study processes at the molecular level has been to carry out investigations with single biomolecular building blocks like pyrimidine as model molecules. Electron attachment to single pyrimidine, as well as to pure clusters and hydrated clusters, was investigated in this study. In striking contrast to the situation with isolated molecules and hydrated clusters, where no anionic monomer is detectable, we were able to observe the molecular anion for the pure clusters. Furthermore, there is evidence that solvation effectively prevents the ring fragmentation of pyrimidine after electron capture. © 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    No preview · Article · Jun 2015 · Angewandte Chemie International Edition
  • [Show abstract] [Hide abstract]
    ABSTRACT: We present high-resolution measurements of the dissociative electron attachment (DEA) to isolated gas-phase hypoxanthine (C5H4N4O, Hyp), a tRNA purine base. The anion mass spectra and individual ion efficiency curves from Hyp were measured as a function of electron energy below 9 eV. The mass spectra at 1 and 6 eV exhibit the highest anion yields, indicating possible common precursor ions that decay into the detectable anionic fragments. The (Hyp − H) anion (C5H3N4O−) exhibits a sharp resonant peak at 1 eV, which we tentatively assign to a dipole-bound state of the keto-N1H,N9H tautomer in which dehydrogenation occurs at either the N1 or N9 position based upon our quantum chemical computations (B3LYP/6-311+G(d,p) and U(MP2-aug-cc-pVDZ+)) and prior studies with adenine. This closed-shell dehydrogenated anion is the dominant fragment formed upon electron attachment, as with other nucleobases. Seven other anions were also observed including (Hyp − NH)−, C4H3N4 −/C4HN3O−, C4H2N3 −, C3NO−/HC(HCN)CN−, OCN−, CN−, and O−. Most of these anions exhibit broad but weak resonances between 4 and 8 eV similar to many analogous anions from adenine. The DEA to Hyp involves significant fragmentation, which is relevant to understanding radiation damage of biomolecules.
    No preview · Article · Jun 2015 · The Journal of Chemical Physics
  • [Show abstract] [Hide abstract]
    ABSTRACT: Die Wechselwirkung niederenergetischer Elektronen mit Biomolekülen spielt eine wichtige Rolle bei der strahlungsinduzierten Mutation von biologischem Gewebe auf molekularer Ebene. Bei Elektronenenergien unter 15 eV ist die dissoziative Elektronenanlagerung einer der Hauptprozesse, die chemische Umwandlungen von Molekülen induzieren. Bisherige Gasphasenstudien hinsichtlich der ablaufenden Prozesse auf molekularer Ebene wurden mit einzelnen Biomolekülbausteinen wie Pyrimidin als Modellmolekül durchgeführt. Hier wurde die Elektronenanlagerung sowohl an einzelnen Pyrimidinmolekülen als auch reinen oder hydratisierten Clustern untersucht. Im Unterschied zum isolierten und hydratisierten Fall, in denen kein anionisches Monomer detektiert werden konnte, gelang es, das Molekülanion bei reinen Clustern zu beobachten. Zusätzlich gibt es Hinweise darauf, dass die Fragmentierung des Pyrimidinrings unter Elektroneneinfang durch Solvatisierung effizient unterdrückt wird.
    No preview · Article · Jun 2015 · Angewandte Chemie
  • [Show abstract] [Hide abstract]
    ABSTRACT: Low energy electrons (0 - 8 eV) effectively decompose 4-nitroimidazole (4NI) and the two methylated isomers 1-methyl-5-nitroimidazole and 1-methyl-4-nitroimidazole via dissociative electron attachment (DEA). The involved unimolecular decompositions range from simple bond cleavages (loss of H•, formation of NO2-) to complex reactions possibly leading to a complete degradation of the target molecule (formation of CN-, etc.). At energies below 2 eV the entire rich chemistry induced by DEA is completely quenched by methylation as already demonstrated in a previous Communication (K. Tanzer, L. Feketeová, B. Puschnigg, P. Scheier, E. Illenberger, and S. Denifl, Angew. Chem. Int. E. 53 (2014) 12240). The observation that in 4NI neutral radicals and radical anions are formed via DEA at high efficiency already at threshold (0 eV) may have significant implications for the development of nitroimidazole based radiosensitisers in tumour radiation therapy.
    No preview · Article · May 2015 · The Journal of Physical Chemistry A
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: Adsorption of helium on free, negatively charged fullerenes is studied in this work. Helium nanodroplets have been doped with fullerenes and ionised by electron attachment. For suitable experimental conditions, C−60 and C−70 anions are found to be complexed with a large number of helium atoms. Prominent anomalies in the ion abundances indicate the high stability of the commensurate 1×1 phase in which all hollow adsorption sites are occupied by one atom each. The adsorption energy for an additional helium atom is about 40% less than for atoms in the commensurate layer, similar to our previous findings for fullerene cations and in agreement with theoretical dissociation energies. Similarly, an anomaly in the adsorption energy occurs when 60 helium atoms are attached to C−60 or 65 to C−70. For C60, the anomaly coincides with the one observed for cationic complexes but for C70 it does not. Implications of these features are discussed in light of several theoretical studies of neutral and positively charged helium-fullerene complexes.
    Full-text · Article · May 2015 · Molecular Physics
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: Nitroimidazoles are important compounds with chemotherapeutic applications as antibacterial drugs or as radiosensitizers in radiotherapy. Despite their use in biological applications, little is known about the fundamental properties of these compounds. Understanding the ionization reactions of these compounds is crucial in evaluating the radiosensitization potential and in developing new and more effective drugs. Thus, the present study investigates the decomposition of negative and positive ions of 2-nitroimidazole and 4(5)-nitroimidazole using low- and high-energy Collision-Induced Dissociation (CID) and Electron-Induced Dissociation (EID) by two different mass spectrometry techniques and is supported by quantum chemistry calculations. EID of [M+H]+ leads to more extensive fragmentation than CID and involves many radical cleavages including loss of H• leading to the formation of the radical cation, M•+. The stability (metastable decay) and the fragmentation (high-energy CID) of the radical cation M•+ have been probed in a crossed-beam experiment involving primary electron ionization of the neutral nitroimidazole. Thus, fragments in the EID spectra of [M+H]+ that come from further dissociation of radical cation M•+ has been highlighted. The loss of NO• radical from M•+ is associated with a high Kinetic Energy Release (KER) of 0.98 eV. EID of [M−H]− also leads to additional fragments compared to CID, however, with much lower cross section. Only EID of [M+H]+ leads to a slight difference in the decomposition of 2-nitroimidazole and 4(5)-nitroimidazole.
    Full-text · Article · Apr 2015 · Physical Chemistry Chemical Physics
  • [Show abstract] [Hide abstract]
    ABSTRACT: The effects of interactions between He(-) and clusters of fullerenes in helium nanodroplets are described. Electron transfer from He(-) to (C60)n and (C70)n clusters results in the formation of the corresponding fullerene cluster dianions. This unusual double electron transfer appears to be concerted and is most likely guided by electron correlation between the two very weakly bound outer electrons in He(-). We suggest a mechanism which involves long range electron transfer followed by the conversion of He(+)into He2 (+), where formation of the He-He bond in He2 (+) releases sufficient kinetic energy for the cation and the dianion to escape their Coulombic attraction. By analogy with the corresponding dications, the observation of a threshold size of n ≥ 5 for formation of both (C60)n (2-) and (C70)n (2-) is attributed to Coulomb explosion rather than an energetic constraint. We also find that smaller dianions can be observed if water is added as a co-dopant. Other aspects of He(-) chemistry that are explored include its role in the formation of multiply charged fullerene cluster cations and the sensitivity of cluster dianion formation on the incident electron energy.
    No preview · Article · Mar 2015 · The Journal of Chemical Physics
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: Negative ion formation through dissociative electron attachment to the gas-phase volatile anaesthetics enflurane and isoflurane (C3H2ClF5O) and to two chlorinated ethanes, pentachloroethane (C2HCl5) and hexachloroethane (C2Cl6), have been studied in a crossed electron-molecular beam two sector field mass spectrometer experiment. Anion efficiency curves for the negatively charged fragments have been measured over an electron energy range of approximately 0–17 eV, with an energy resolution of ∼1 eV. For the chlorinated ethanes, resonance features were generally found at ∼ 0 eV and at 7.5 eV. For the volatile anaesthetics, no zero energy resonances were observed. Instead, product anions were detected mainly in the 2–3 eV and 9 eV energy regions, with the exception of Cl-, whose dominant resonance occurs at approximately 0.6 eV and 0.9 eV for isoflurane and enflurane, respectively. To aid in the interpretation of the experimental results, quantum chemical calculations providing thermochemical thresholds of anion formation are also presented.
    Full-text · Article · Feb 2015 · International Journal of Mass Spectrometry
  • Katrin Tanzer · Andrzej Pelc · Stefan E Huber · Z Czupyt · Stephan Denifl
    [Show abstract] [Hide abstract]
    ABSTRACT: Cyanamide (NH2CN) is a molecule relevant for interstellar chemistry and the chemical evolution of life. In the present investigation, dissociative electron attachment to NH2CN has been studied in a crossed electron–molecular beams experiment in the electron energy range from about 0 eV to 14 eV. The following anionic species were detected: NHCN−, NCN−, CN−, NH2 −, NH−, and CH2 −. The anion formation proceeds within two broad electron energy regions, one between about 0.5 and 4.5 eV and a second between 4.5 and 12 eV. A discussion of possible reaction channels for all measured negative ions is provided. The experimental results are compared with calculations of the thermochemical thresholds of the anions observed. For the dehydrogenated parent anion, we explain the deviation between the experimental appearance energy of the anion with the calculated corresponding reaction threshold by electron attachment to the isomeric form of NH2CN—carbodiimide.
    No preview · Article · Jan 2015 · The Journal of Chemical Physics
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: Clusters of sulfur are grown by passing superfluid helium nanodroplets through a pickup cell filled with sulfur vapor. In some experiments the droplets are codoped with C60. The doped droplets are collided with energetic electrons and the abundance distributions of positively and negatively charged cluster ions are recorded. We report, specifically, distributions of Sm+, Sm–, and C60Sm– containing up to 41 sulfur atoms. We also observe complexes of sulfur cluster anions with helium; distributions are presented for HenSm– with n ≤ 31 and m ≤ 3. The similarity between anionic and cationic C60Sm± spectra is in striking contrast to the large differences between spectra of Sm+ and Sm–.
    Full-text · Article · Jan 2015 · The Journal of Physical Chemistry C
  • [Show abstract] [Hide abstract]
    ABSTRACT: The formation of dianions in helium nanodroplets is reported for the first time. The fullerene cluster dianions (C60)n2− and (C70)n2− were observed by mass spectrometry for n≥5 when helium droplets containing the appropriate fullerene were subjected to electron impact at approximately 22 eV. A new mechanism for dianion formation is described, which involves a two-electron transfer from the metastable He− ion. As well as the prospect of studying other dianions at low temperature using helium nanodroplets, this work opens up the possibility of a wider investigation of the chemistry of He−, a new electron-donating reagent.
    No preview · Article · Dec 2014 · Angewandte Chemie
  • Source
    Matthias Daxner · Stephan Denifl · Paul Scheier · Andrew M. Ellis
    [Show abstract] [Hide abstract]
    ABSTRACT: The self-assembly of salt nanocrystals from chemical reactions inside liquid helium is reported for the first time. Reaction is initiated by an electron impacting a helium nanodroplet containing sodium atoms and SF6 molecules, leading to preferential production of energetically favorable structures based on the unit cell of crystalline NaF. These favorable structures are observed as magic number ions (anomalously intense peaks) in mass spectra and are seen in both cationic and anionic channels in mass spectra, for example, (NaF)nNa+ and (NaF)nF−. In the case of anions the self-assembly is not directly initiated by electrons: the dominant process involves resonant electron-induced production of metastable electronically excited He− anions, which then initiate anionic chemistry by electron transfer.
    Preview · Article · Dec 2014 · Angewandte Chemie International Edition
  • Matthias Daxner · Stephan Denifl · Paul Scheier · Andrew M. Ellis
    [Show abstract] [Hide abstract]
    ABSTRACT: The self-assembly of salt nanocrystals from chemical reactions inside liquid helium is reported for the first time. Reaction is initiated by an electron impacting a helium nanodroplet containing sodium atoms and SF6 molecules, leading to preferential production of energetically favorable structures based on the unit cell of crystalline NaF. These favorable structures are observed as magic number ions (anomalously intense peaks) in mass spectra and are seen in both cationic and anionic channels in mass spectra, for example, (NaF)nNa+ and (NaF)nF−. In the case of anions the self-assembly is not directly initiated by electrons: the dominant process involves resonant electron-induced production of metastable electronically excited He− anions, which then initiate anionic chemistry by electron transfer.
    No preview · Article · Dec 2014 · Angewandte Chemie
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: Niederenergetische Elektronen (NEEs) mit Energien unter 2 eV zersetzen 4-Nitroimidazol (4NI) sehr effizient über dissoziative Elektronenanlagerung (DEA). Diese Reaktionen umfassen einfache Bindungsbrüche, aber auch komplexere Prozesse wie mehrfache Bindungsbrüche und die Bildung neuer Moleküle. Alle diese Reaktionen zeigen sich als scharfe Strukturen im detektierten Anionensignal und werden als Feshbach-Resonanzen gedeutet, die effiziente Wegbereiter für DEA sind. Die außergewöhnlich vielfältigen chemischen Reaktionen von 4NI werden bei Methylierung von 4NI an der N1-Position komplett blockiert. Diese bemerkenswerten Resultate haben auch Auswirkung auf die Entwicklung von Radiosensibilisatoren für die Strahlungstherapie von Tumoren auf Basis von Nitroimidazol.
    Preview · Article · Nov 2014 · Angewandte Chemie
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: Low-energy electrons (LEEs) at energies of less than 2 eV effectively decompose 4-nitroimidazole (4NI) by dissociative electron attachment (DEA). The reactions include simple bond cleavages but also complex reactions involving multiple bond cleavages and formation of new molecules. Both simple and complex reactions are associated with pronounced sharp features in the anionic yields, which are interpreted as vibrational Feshbach resonances acting as effective doorways for DEA. The remarkably rich chemistry of 4NI is completely blocked in 1-methyl-4-nitroimidazole (Me4NI), that is, upon methylation of 4NI at the N1 site. These remarkable results have also implications for the development of nitroimidazole based radiosensitizers in tumor radiation therapy.
    Preview · Article · Nov 2014 · Angewandte Chemie International Edition

Publication Stats

3k Citations
527.75 Total Impact Points

Institutions

  • 2002-2015
    • University of Innsbruck
      • Institute for Ion Physics and Applied Physics
      Innsbruck, Tyrol, Austria
  • 2013
    • University of Birmingham
      Birmingham, England, United Kingdom
  • 2012
    • Freie Universität Berlin
      • Institute of Theoretical Physics
      Berlín, Berlin, Germany
  • 2007
    • University of Iceland
      • Science Institute
      Reikiavik, Capital Region, Iceland