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ABSTRACT: Significant increase of the ion yield at m/z 27 in collisions of low-energy ions of N2+ and N+ with hydrocarbon-covered room-temperature or heated surfaces of tungsten, carbon-fiber-composite, and beryllium, not observed in analogous collisions of Ar+, is ascribed to formation of HCN+ in heterogeneous reactions between N2+ or N+ and surface hydrocarbons. The formation of HCN+ in the reaction with N+ indicated an exothermic reaction with no activation barrier, likely to occur even at very low collision energies. In the reaction with N2+, the formation of HCN+ was observed to a different degree on these room-temperature and heated (150°C and 300°C) surfaces at incident energies above about 50eV. This finding suggested an activation barrier or reaction endothermicity of the heterogeneous reaction of about 3-3.5 eV. The main process in N2+ or N+ interaction with the surfaces is ion neutralization; the probability of forming the reaction product HCN+ was very roughly estimated for both N2+ and N+ ions to about one in 104 collisions with the surfaces.
The Journal of Physical Chemistry A 04/2013; · 2.95 Impact Factor
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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: Gas phase dissociative electron attachment (DEA) measurements with methyl-dialanine, C(7)H(14)N(2)O(3), are performed in a crossed electron-molecular beam experiment at high energy resolution (∼120 meV). Anion efficiency yields as a function of the incident electron energy are obtained for the most abundant fragments up to electron energies of ∼15 eV. There is no evidence of molecular anion formation whereas the dehydrogenated closed shell anion (M-H)(-) is one of the most dominant reaction products. Quantum chemical calculations are performed to investigate the electron attachment process and to elucidate site selective bond cleavage in the (M-H)(-) DEA-channel. Previous DEA studies on dialanine have shown that (M-H)(-) formation proceeds through abstraction of a hydrogen atom from the carboxyl and amide groups, contributing to two distinct resonances at 0.81 and 1.17 eV, respectively [D. Gschliesser, V. Vizcaino, M. Probst, P. Scheier and S. Denifl, Chem.-Eur. J., 2012, 18, 4613-4619]. Here we show that by methylation of the carboxyl group, all (calculated) thresholds for H-loss from the different sites in the dialanine molecule are shifted up to a maximum of 1.4 eV. The lowest lying resonance observed experimentally for (M-H)(-) remains operative from the amide group at the electron energy of 2.4 eV due to the methylation. We further study methylation-induced effects on the unimolecular dissociation leading to a variety of negatively charged DEA products.
Physical Chemistry Chemical Physics 02/2013; 15(11):3834-40. · 3.57 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: High-resolution mass spectra of helium nanodroplets doped with hydrogen or deuterium reveal that copious amounts of helium can be bound to H(+) , H(2) (+) , H(3) (+) , and larger hydrogen-cluster ions. All conceivable He(n) H(x) (+) stoichiometries are identified if their mass is below the limit of ≈120 u set by the resolution of the spectrometer. Anomalies in the ion yields of He(n) H(x) (+) for x=1, 2, or 3, and n≤30 reveal particularly stable cluster ions. Our results for He(n) H(1) (+) are consistent with conclusions drawn from previous experimental and theoretical studies which were limited to smaller cluster ions. The He(n) H(3) (+) series exhibits a pronounced anomaly at n=12 which was outside the reliable range of earlier experiments. Contrary to findings reported for other diatomic dopant molecules, the monomer ion (i.e. H(2) (+) ) retains helium with much greater efficiency than hydrogen-cluster ions.
ChemPhysChem 10/2012; · 3.41 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: Helium atoms bind strongly to alkali cations which, when embedded in liquid helium, form so-called snowballs. Calculations suggest that helium atoms in the first solvation layer of these snowballs form rigid structures and that their number (n) is well defined, especially for the lighter alkalis. However, experiments have so far failed to accurately determine values of n. We present high-resolution mass spectra of Na(+)He(n), K(+)He(n), Na(2)(+)He(n) and K(2)(+)He(n), formed by electron ionization of doped helium droplets; the data allow for a critical comparison with several theoretical studies. For sodium and potassium monomers the spectra indicate that the value of n is slightly smaller than calculated. Na(2)(+)He(n) displays two distinct anomalies at n=2 and n=6, in agreement with theory; dissociation energies derived from experiment closely track theoretical values. K(2)(+)He(n) distributions are fairly featureless, which also agrees with predictions.
Chemistry 02/2012; 18(14):4411-8. · 5.93 Impact Factor
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ABSTRACT: The dehydrogenated parent anion [M-H](-) is one of the most dominant anions formed in dissociative electron attachment to various small biomolecules like nucleobases and single amino acids. In the present study, we investigate the [M-H](-) channel for the dipeptide dialanine by utilizing an electron monochromator and a two-sector-field mass spectrometer. At electron energies below 2 eV, the measured high-resolution ion-efficiency curve has a different shape to that for the single amino acid alanine, which is explained by the altered threshold energies for formation of [M-H](-) determined in quantum chemical calculations. Moreover, the structure of the formed [M-H](-) anion is further studied by investigating the unimolecular and collision-induced decay of this anion. Trajectory calculations have been carried out to aid the interpretation of the experimentally observed fragmentation patterns.
Chemistry 02/2012; 18(15):4613-9. · 5.93 Impact Factor
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Christian Leidlmair,
Yang Wang,
Peter Bartl,
Harald Schöbel,
Stephan Denifl,
Michael Probst,
Manuel Alcamí,
Fernando Martín,
Henning Zettergren,
Klavs Hansen,
Olof Echt, Paul Scheier
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ABSTRACT: Helium adsorbed on C(60)(+) and C(70)(+) exhibits phenomena akin to helium on graphite. Mass spectra suggest that commensurate layers form when all carbon hexagons and pentagons are occupied by one He each, but that the solvation shell does not close until 60 He atoms are adsorbed on C(60)(+), or 62 on C(70)(+). Molecular dynamics simulations of C(60)He(n)(+) at 4 K show that the commensurate phase is solid. Helium added to C(60)He(32)(+) will displace some atoms from pentagonal sites, leading to coexistence of a registered layer of immobile atoms interlaced with a nonregistered layer of mobile atoms.
Physical Review Letters 02/2012; 108(7):076101. · 7.37 Impact Factor
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ABSTRACT: The electron ionization of helium droplets doped with methane clusters is investigated for the first time using high-resolution mass spectrometry. The dominant ion products ejected into the gas phase are the unprotonated (CH(4))(n)(+) cluster ions along with the protonated ions, CH(5)(+)(CH(4))(n-1). The mass spectra show clear evidence for magic numbers, which are broadly consistent with icosahedral shell closings. However, unusual features were observed, including different magic numbers for CH(5)(+)(CH(4))(n-1) (n=55, 148) when compared to (CH(4))(n)(+) (n=54, 147). Possible interpretations for some of these differences are proposed. Products of the type [C(2)H(x)(CH(4))(n)](+), which result from ion-molecule chemistry, are also observed and these too show clear magic number features. Finally, we report the first observation of (CH(4))(n)(2+) dications from methane clusters. The threshold for dication survival occurs at n≥70 and is in good agreement with a liquid droplet model for fission of multiply charged ions. Furthermore, we present evidence showing that these dications are formed by an unusual two-step mechanism which is initiated by charge transfer to generate a monocation and is then followed by Penning ionization to generate a dication.
ChemPhysChem 12/2011; 13(2):469-76. · 3.41 Impact Factor
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ABSTRACT: Gas phase studies of dissociative electron attachment to simple alkyl (CF(3)SO(3)CH(3)) and aryl (C(6)H(5)SO(3)CF(3) and CF(3)SO(3)C(6)H(4)CH(3)) triflates, model molecules of nonionic photoacid generators for modern lithographic applications, were performed. The fragmentation pathways under electron impact below 10 eV were identified by means of crossed electron-molecular beam mass spectrometry. Major dissociation channels involved C-O, S-O, or C-S bond scissions in the triflate moiety leading to the formation of triflate (OTf(-)), triflyl (Tf(-)), or sulfonate (RSO(3)(-)) anions, respectively. A resonance leading to C-O bond breakage and OTf(-) formation in alkyl triflates occurred at electron energies about 0.5 eV lower than the corresponding resonance in aryl triflates. A resonance leading to S-O bond breakage and Tf(-) formation in aryl triflates occurred surprisingly at the same electron energies as C-O bond breakage. In case of alkyl triflates S-O bond breakage required 1.4 eV higher electron energies to occur and proceeded with substantially lower yields than in aryl triflates. C-S bond scission occurred for all presently studied triflates at energies close to 3 eV.
The Journal of chemical physics 12/2011; 135(21):214309. · 3.09 Impact Factor
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ABSTRACT: The possible contribution of fullerenes and their derivatives to the diffuse interstellar bands (DIBs) has been discussed for some time. Bare fullerenes have been identified in the interstellar medium but they explain few, if any, of the DIBs. In this contribution we show that C-60(+) and C-70(+) will physisorb copious amounts of molecular hydrogen at low temperature. H-2 forms an ordered layer around the fullerene ion; the first coordination shell closes when each carbon ring is decorated with one H-2. We estimate that fullerenes in dense clouds may very well be complexed with H-2. The occurrence of H-2-fullerene complexes in translucent clouds is less likely but cannot be ruled out. They are unlikely to occur in diffuse clouds, which are the major sources of DIBs.
The Astrophysical Journal Letters. 11/2011; 738(1-1):L4.
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International Journal of Mass Spectrometry 09/2011; · 2.55 Impact Factor
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ABSTRACT: The submersion of sodium clusters beyond a critical size in helium nanodroplets, which has recently been predicted on theoretical grounds, is demonstrated for the first time. Confirmation of a clear transition from a surface location, which occurs for alkali atoms and small clusters, to full immersion for larger clusters, is provided by identifying the threshold electron energy required to initiate Na(n) cluster ionization. On the basis of these measurements, a lower limit for the cluster size required for submersion, n ≥ 21, has been determined. This finding is consistent with the recent theoretical prediction.
The Journal of chemical physics 07/2011; 135(4):044309. · 3.09 Impact Factor
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ABSTRACT: A detailed study on dissociative electron attachment (DEA) to β-alanine (βA) in the gas phase is presented. Ion yields as a function of the incident electron energy from about 0 to 15 eV have been measured for most of the fragments. As for all α-amino acids, the main reaction corresponds to the loss of a hydrogen atom, although many other fragments have been observed that involved more complex bond cleavages. Threshold energies have been calculated by using the G4(MP2) method for various decomposition reactions. Fragmentation pathways were also investigated to measure metastable decays of the intermediate fragment anion (βA-H)(-) by using the mass-analyzed ion kinetic energy (MIKE) scan technique. Comparisons with α-alanine and other amino acids are made when relevant.
ChemPhysChem 05/2011; 12(7):1272-9. · 3.41 Impact Factor
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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: Helium nanodroplets are co-doped with C(60) and ammonia. Mass spectra obtained by electron ionization reveal cations containing ammonia clusters complexed with up to four C(60) units. The high mass resolution of Δm/m≈ 1/6000 makes it possible to separate the contributions of protonated, unprotonated and dehydrogenated ammonia. C(60) aggregates suppress the proton-transfer reaction which usually favors the appearance of protonated ammonia cluster ions. Unprotonated C(x)(NH(3))(n)(+) ions (x = 60, 120, 180) exceed the abundance of the corresponding protonated ions if n < 5; for larger values of n the abundances of C(60)(NH(3))(n)(+) and C(60)(NH)(n-1)NH(4)(+) become about equal. Dehydrogenated C(60)NH(2)(+) ions are relatively abundant; their formation is attributed to a transient doubly charged C(60)-ammonia complex which forms either by an Auger process or by Penning ionization following charge transfer between the primary He(+) ion and C(60). The abundance of C(x)NH(3)(+) and C(x)NH(4)(+) ions (x = 120 or 180) is one to two orders of magnitude weaker than the abundance of ions containing one or two additional ammonia molecules. However, a model involving evaporation of NH(3) or NH(4) from the presumably weakly bound C(x)NH(3)(+) and C(x)NH(4)(+) ions is at odds with the lack of enhancement in the abundance of C(120)(+) and C(180)(+). Mass spectra of C(60) dimers complexed with water complement a previous study of C(60)(H(2)O)(n)(+) recorded at much lower mass resolution.
Physical Chemistry Chemical Physics 11/2010; 13(3):1092-8. · 3.57 Impact Factor
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ABSTRACT: Electron attachment to clusters of N(2)O in the energy range of 0-4 eV yields the ionic complexes [(N(2)O)(n)O](-), [(N(2)O)(n)NO](-), and (N(2)O)(n) (-) . The shape of the ion yields of the three homologous series differs substantially reflecting the different formation mechanisms. While the generation of [(N(2)O)(n)O](-) can be assigned to dissociative electron attachment (DEA) of an individual N(2)O molecule in the target cluster, the formation of [(N(2)O)(n)NO](-) is interpreted via a sequence of ion molecule reactions involving the formation of O(-) via DEA in the first step. The nondecomposed complexes (N(2)O)(n) (-) are preferentially formed at very low energies (below 0.5 eV) as a result of intramolecular stabilization of a diffuse molecular anion at low energy. The ion yields of [(N(2)O)(n)O](-) and (N(2)O)(n) (-) versus electron energy show sharp peaks at the threshold region, which can be assigned to vibrational Feshbach resonances mediated by the diffuse anion state as already observed in an ultrahigh resolution electron attachment study of N(2)O clusters [E. Leber, S. Barsotti, J. Bömmels, J. M. Weber, I. I. Fabrikant, M.-W. Ruf, and H. Hotop, Chem. Phys. Lett. 325, 345 (2000)].
The Journal of chemical physics 10/2010; 133(15):154512. · 3.09 Impact Factor
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Achim Edtbauer,
Stephan Denifl,
Violaine Vizcaino,
Lukas An der Lan,
Katherine Russell,
Jörg Taubitz,
Uta Wille,
Linda Feketeova,
Richard A J O'Hair,
Tilmann D Märk,
Eugen Illenberger, Paul Scheier
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ABSTRACT: Electrons with virtually no kinetic energy (close to 0 eV) trigger the decomposition of cytotoxic cyclobutane-pyrimidine dimer (CPD) into a surprisingly large variety of fragment ions plus their neutral counterparts. The response of CPD to low energy electrons is thus comparable to that of explosives like trinitrotoluene (TNT). The dominant unimolecular reaction is the splitting into two thymine like units, which can be considered as the essential molecular step in the photolyase of CPD. We find that CPD is significantly more sensitive towards low energy electrons than its thymine building blocks. It is proposed that electron attachment at very low energy proceeds via dipole bound states, supported by the large dipole moment of the molecule (6.2 D). These states act as effective doorways to dissociative electron attachment (DEA).
ChemPhysChem 02/2010; 11(3):561-4. · 3.41 Impact Factor
