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ABSTRACT: Helium droplets are doped with fullerenes (either C60 or C70) and hydrogen (H2 or D2) and investigated by high-resolution mass spectrometry. In addition to pure helium and hydrogen cluster ions, hydrogen-fullerene complexes are observed upon electron ionization. The composition of the main ion series is (H2)nHCm + where m = 60 or 70. Another series of even-numbered ions, (H2)nCm +, is slightly weaker in stark contrast to pure hydrogen cluster ions for which the even-numbered series (H2)n + is barely detectable. The ion series (H2)nHCm + and (H2)nCm + exhibit abrupt drops in ion abundance at n = 32 for C60 and 37 for C70, indicating formation of an energetically favorable commensurate phase, with each face of the fullerene ion being covered by one adsorbate molecule. However, the first solvation layer is not complete until a total of 49 H2 are adsorbed on C60 +; the corresponding value for C70 + is 51. Surprisingly, these values do not exhibit a hydrogen-deuterium isotope effect even though the isotope effect for H2∕D2 adsorbates on graphite exceeds 6%. We also observe doubly charged fullerene-deuterium clusters; they, too, exhibit abrupt drops in ion abundance at n = 32 and 37 for C60 and C70, respectively. The findings imply that the charge is localized on the fullerene, stabilizing the system against charge separation. Density functional calculations for C60-hydrogen complexes with up to five hydrogen atoms provide insight into the experimental findings and the structure of the ions. The binding energy of physisorbed H2 is 57 meV for H2C60 + and (H2)2C60 +, and slightly above 70 meV for H2HC60 + and (H2)2HC60 +. The lone hydrogen in the odd-numbered complexes is covalently bound atop a carbon atom but a large barrier of 1.69 eV impedes chemisorption of the H2 molecules. Calculations for neutral and doubly charged complexes are presented as well.
The Journal of chemical physics 02/2013; 138(7):074311. · 3.09 Impact Factor
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ABSTRACT: Electron ionization of three perfluoroethers (PFEs), C(6) F(14) O(3) , C(8) F(18) O(4) , and C(10) F(20) O(5) , is studied in the gas phase and when the molecules are embedded in ultracold helium (He) droplets. The molecules investigated are model compounds for perfluoropolyethers used as lubricants in technical applications. The present study gives insight into possible radiolysis pathways upon radiation exposure.
The experiments utilized a crossed electron/droplet beam apparatus consisting of a He droplet source and pick-up chamber combined with a commercial time-of-flight mass spectrometer. The doped droplets were ionized by electron ionization at 70 eV.
The He environment strongly affects the ionization patterns in the way that both the molecular ion M(+) and high-mass fragment ions formed by the loss of light neutral species such as F ([M-F](+) ), or CF(3) OCF(2) ([M-CF(3) OCF(2) ](+) ), etc., became strong signals in the mass spectrum. These signals were not or only barely visible in the gas-phase experiment and were identified as short lived (< µs) dissociation intermediates which in the gas phase immediately decomposed into lower-mass fragment ions.
Ionic fragmentation intermediates are frozen and subsequently stabilized in the He environment. Helium droplets can hence be viewed as a cryogenic laboratory transforming short-lived decomposition intermediates into stable fragment ions appearing as strong signals in the mass spectrum. Copyright © 2012 John Wiley & Sons, Ltd.
Rapid Communications in Mass Spectrometry 01/2013; 27(2):298-304. · 2.79 Impact Factor
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ABSTRACT: Low-energy secondary electrons are formed when energetic particles interact with matter. High-energy electrons or ions are used to form metallic structures from adsorbed organometallic molecules like W(CO)(6) on surfaces. We investigated low-energy electron attachment to W(CO)(6) in the gas phase to elucidate possible reactions during surface modification.
Two crossed electron/molecular beam setups were utilised: (i) a high-resolution electron monochromator combined with a quadrupole mass spectrometer which was used for the measurement of relative cross sections as a function of the electron energy, and (ii) a double focusing mass spectrometer used for measurements of metastable decays of anions.
The study was performed in the electron energy range between ~0 and 14 eV. W(CO)(6) efficiently decomposed upon attachment of a low-energy electron and no stable W(CO)(6)(-) anion was observed on mass spectrometric time scales. The transient negative ion formed lost instead sequentially CO ligands. The fragment anions W(CO)(5)(-), W(CO)(4)(-), W(CO)(3)(-), and W(CO)(2)(-) were observed. However, no W(-) was detectable.
Dissociative electron attachment (DEA) to W(CO)(6) led to strong dissociation but a complete loss of all CO ligands was not observed in DEA. Deposit contaminations might be a direct result of DEA reactions close to the irradiation spot in beam deposition techniques.
Rapid Communications in Mass Spectrometry 09/2012; 26(17):2093-8. · 2.79 Impact Factor
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ABSTRACT: Helium nanodroplets, formed in a supersonic expansion, are doped with C60 in a pickup cell. In some experiments, they are co-doped with water. Electrons are attached to the doped droplets; the yield
of anions is recorded as a function of electron energy. The C60
- yield extends to much higher energies than in experiments involving isolated, hot fullerenes; we attribute the difference
to the low temperature of the neutral precursors and the efficient cooling of the nascent anions by the helium droplet, which
quench thermally activated autodetachment. The yields of (C60)2
- and C60D2O- anions reveal another important factor, namely depletion of the anion signal by dissociation which is energetically more
facile than autodetachment.
The European Physical Journal D 04/2012; 52(1):91-94. · 1.48 Impact Factor
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ABSTRACT: Mass spectra of doped and undoped helium droplets are presented. The high resolution of
the time-of-flight spectrometer (m/Δm≅5000) makes it
possible to fully resolve small helium cluster ions from impurities and to unambiguously
identify abundance anomalies in the size distribution of
He
n
+.
The yield of He4
+
shows the well-known enhancement relative to other small cluster ions when the expansion
changes from sub- to supercritical, provided the electron energy exceeds a value of
40±1eV, the threshold for formation of electronically excited ions. Upon doping with
krypton, pure Kr
n
+
cluster ions containing up to 41 Kr atoms are observed. The spectra exhibit abundance
anomalies at 13, 16, 19, 22 & 23, 26 and 29, in agreement with spectra obtained by
ionization of bare krypton clusters that are formed in neat supersonic beams. Mixed
clusters He
m
Kr+ indicate closure of a solvation
shell at m=12.
The European Physical Journal D 04/2012; 63(2):209-214. · 1.48 Impact Factor
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ABSTRACT: Here we report electron impact ionization studies with the amino acid valine in different environments, i.e., (i) isolated
in the gas phase, (ii) embedded in superfluid helium droplets and (iii) co-embedded with water in superfluid helium droplets.
Mass spectra are presented for all three environments for which changes in the fragmentation pattern of valine upon ionization
are investigated. Comparison is made with previous electron impact ionization and photoionization studies with valine in the
gas phase which confirms the fragile nature of this amino acid. Embedding valine in cold superfluid helium droplets leads
to the formation of highly abundant protonated valine clusters. Co-embedding water with valine in helium droplets reduces
fragmentation of valine.
The European Physical Journal D 04/2012; 51(1):73-79. · 1.48 Impact Factor
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ABSTRACT: The degradation of oligonucleotide films containing differing base sequences induced by 4keV C+ions has been studied experimentally. The oligonucleotides were deposited onto a gold coated stainless steel substrate and
the anions and cations released from the films were analyzed by a quadrupole mass spectrometer. The total ion desorption yield
was recorded as a function of time using a constant C+ ion flux of
6 × 1014ionscm-2 s-1. At low fluences the formation of small ionic fragments was observed, whilst for fluences greater than
1.2 × 1018ionscm-2 molecules were sputtered from the substrate. In addition to studies of the influence of a particular base to the total cation
desorption yield, the effect of base substitution with bromine was measured for negative ion desorption. These results showed
a strong degradation of oligonucleotide films during ion bombardment.
The European Physical Journal D 04/2012; 60(1):59-63. · 1.48 Impact Factor
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ABSTRACT: We present the results obtained in dissociative electron attachment to Semtex in the gas phase by making use of a crossed electron-molecular beam experiment. Energy dependence of partial cross sections has been measured in an electron energy range from 0 to 12 eV with an energy resolution of similar to 80 meV. Semtex 1A and H are mixtures of two other explosives, i.e., royal demolition explosive - RDX (hexogen) and pentaerythritol tetranitrate - PETN (pentrite), appearing in different fractions. In fresh samples product anions from both explosives can be detected together with anions originating from volatile markers. After some time we only observe product ions formed upon dissociative electron attachment to PETN, which are not observed for a mixture of pure RDX and PETN. For both Semtex samples, an additional anionic feature at 210 m/z can be assigned to the binder butyl rubber. (C) 2011 Elsevier B.V. All rights reserved.
International Journal of Mass Spectrometry. 01/2012; 309:39-43.
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ABSTRACT: Results of a detailed study on electron interactions with nitromethane (CH(3)NO(2)) embedded in helium nanodroplets are reported. Anionic and cationic products formed are analysed by mass spectrometry. When the doped helium droplets are irradiated with low-energy electrons of about 2 eV kinetic energy, exclusively parent cluster anions (CH(3)NO(2))(n)(-) are formed. At 8.5 eV, three anion cluster series are observed, i.e., (CH(3)NO(2))(n)(-), [(CH(3)NO(2))(n)-H](-), and (CH(3)NO(2))(n)NO(2)(-), the latter being the most abundant. The results obtained for anions are compared with previous electron attachment studies with bare nitromethane and nitromethane condensed on a surface. The cation chemistry (induced by electron ionization of the helium matrix at 70 eV and subsequent charge transfer from He(+) to the dopant cluster) is dominated by production of methylated and protonated nitromethane clusters, (CH(3)NO(2))(n)CH(3)(+) and (CH(3)NO(2))(n)H(+).
The Journal of chemical physics 11/2011; 135(17):174504. · 3.09 Impact Factor
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ABSTRACT: Negative ion formation in the three perfluoroethers (PFEs) diglyme (C(6)F(14)O(3)), triglyme (C(8)F(18)O(4)) and crownether (C(10)F(20)O(5)) is studied following electron attachment in the range from ∼0 to 15 eV. All three compounds show intense low energy resonances at subexcitation energies (<3 eV) decomposing into a variety of negatively charged fragments. These fragment ions are generated via dissociative electron attachment (DEA), partly originating from sequential decompositions on the metastable (μs) time scale as observed from the MIKE (metastable induced kinetic energy) scans. Only in perfluorocrownether a signal due to the non-decomposed parent anion is observed. Additional and comparatively weaker resonances are located in the energy range between ∼10 and 17 eV which preferentially decompose into lighter ions. It is suggested that specific features of perfluoropolyethers (PFPEs) relevant in applications, e.g., the strong bonding to surfaces induced by UV radiation of the substrate or degradation of PFPE films in computer hard disc drives can be explained by their pronounced sensitivity towards low energy electrons.
International Journal of Mass Spectrometry 09/2011; 306(1):63-69. · 2.55 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. H2 forms an ordered layer around the fullerene ion; the first coordination shell closes when each carbon ring is decorated with one H2. We estimate that fullerenes in dense clouds may very well be complexed with H2. The occurrence of H2-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 08/2011; 738(1):L4. · 5.53 Impact Factor
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ABSTRACT: Dissociative electron attachment (DEA) to gaseous formamide, HCONH(2), has been investigated in the energy range between 0 eV and 18 eV using a crossed electron/molecule beam technique. The negative ion fragments have been comprehensively monitored and assigned to molecular structures by comparison with the results for two differently deuterated derivatives, namely 1D-formamide, DCONH(2), and N,N,D-formamide, HCOND(2). The following products were observed: HCONH(-), CONH(2)(-), HCON(-), OCN(-), HCNH(-), CN(-), NH(2)(-)/O(-), NH(-), and H(-). NH(2)(-) was also separated from O(-) by using high-resolution negative ion mass spectrometry. Four resonant dissociation channels can be resolved, the strongest ones being located between 2.0 and 2.7 eV and between 6.0 and 7.0 eV. CN(-) as the most abundant fragment and HCONH(-) are the dominant products of the first of these two resonances. The most important products of the latter resonance are NH(2)(-), CN(-), H(-), CONH(2)(-), and OCN(-). It is thus found that the loss of neutral H is a site-selective process, dissociation from the N site taking place between 2.0 and 2.7 eV while dissociation from the C site occurs between 6.0 and 7.0 eV. The suitability of these reactions and thus of formamide as an agent for electron-induced surface functionalisation is discussed.
Physical Chemistry Chemical Physics 07/2011; 13(26):12305-13. · 3.57 Impact Factor
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ABSTRACT: New measurements of the absolute cross sections for dissociative electron attachment (DEA) in molecular hydrogen and deuterium are presented which resolve previous ambiguities and provide a test bed for theory. The experimental methodology is based upon a momentum imaging time-of-flight spectrometer that allowed us to eliminate any contributions due to electronically excited metastable neutrals and ultraviolet light while ensuring detection of all the ions. The isotope effect in the DEA process in the two molecules is found to be considerably larger than previously observed. More importantly, it is found to manifest in the polar dissociation process (also known as ion pair production) as well.
Physical Review Letters 06/2011; 106(24):243201. · 7.37 Impact Factor
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ABSTRACT: Attachment of free, low-energy electrons to dinitrobenzene (DNB) in the gas phase leads to DNB(-) as well as several fragment anions. DNB(-), (DNB-H)(-), (DNB-NO)(-), (DNB-2NO)(-), and (DNB-NO(2))(-) are found to undergo metastable (unimolecular) dissociation. A rich pattern of resonances in the yield of these metastable reactions versus electron energy is observed; some resonances are highly isomer-specific. Most metastable reactions are accompanied by large average kinetic energy releases (KER) that range from 0.5 to 1.32 eV, typical of complex rearrangement reactions, but (1,3-DNB-H)(-) features a resonance with a KER of only 0.06 eV for loss of NO. (1,3-DNB-NO)(-) offers a rare example of a sequential metastable reaction, namely, loss of NO followed by loss of CO to yield C(5)H(4)O(-) with a large KER of 1.32 eV. The G4(MP2) method is applied to compute adiabatic electron affinities and reaction energies for several of the observed metastable channels.
The Journal of chemical physics 12/2010; 133(24):244302. · 3.09 Impact Factor
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S Denifl,
F Zappa,
I Mähr,
A Mauracher,
M Probst,
J Urban,
P Mach,
A Bacher,
D K Bohme,
O Echt,
T D Märk,
P Scheier
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ABSTRACT: Water clusters are known to undergo an autoprotonation reaction upon ionization by photons or electron impact, resulting in the formation of (H(2)O)(n)H(3)O(+). Ejection of OH cannot be quenched by near-threshold ionization; it is only partly quenched when clusters are complexed with inert gas atoms. Mass spectra recorded by electron ionization of water-doped helium droplets show that the helium matrix also fails to quench OH loss. The situation changes drastically when helium droplets are codoped with C(60). Charged C(60)-water complexes are predominantly unprotonated; C(60)(H(2)O)(4)(+) and (C(60))(2)(H(2)O)(4)(+) appear with enhanced abundance. Another intense ion series is due to C(60)(H(2)O)(n)OH(+); dehydrogenation is proposed to be initiated by charge transfer between the primary He(+) ion and C(60). The resulting electronically excited C(60)(+*) leads to the formation of a doubly charged C(60)-water complex either via emission of an Auger electron from C(60)(+*), or internal Penning ionization of the attached water complex, followed by charge separation within {C(60)(H(2)O)(n)}(2+). This mechanism would also explain previous observations of dehydrogenation reactions in doped helium droplets. Mass-analyzed ion kinetic energy scans reveal spontaneous (unimolecular) dissociation of C(60)(H(2)O)(n)(+). In addition to the loss of single water molecules, a prominent reaction channel yields bare C(60)(+) for sizes n=3, 4, or 6. Ab initio Hartree-Fock calculations for C(60)-water complexes reveal negligible charge transfer within neutral complexes. Cationic complexes are well described as water clusters weakly bound to C(60)(+). For n=3, 4, or 6, fissionlike desorption of the entire water complex from C(60)(H(2)O)(n)(+) energetically competes with the evaporation of a single water molecule.
The Journal of chemical physics 06/2010; 132(23):234307. · 3.09 Impact Factor
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ABSTRACT: The first detailed study of electron attachment to amino acid clusters is reported. The amino acids chosen for investigation were glycine, alanine, and serine. Clusters of these amino acids were formed inside helium nanodroplets, which provide a convenient low temperature (0.37 K) environment for growing noncovalent clusters. When subjected to low energy (2 eV) electron impact the chemistry for glycine and alanine clusters was found to be similar. In both cases, parent cluster anions were the major products, which contrasts with the corresponding monomers in the gas phase, where the dehydrogenated products ([AA(n)-H](-), where AA = amino acid monomer) dominate. Serine clusters are different, with the major product being the parent anion minus an OH group, an outcome presumably conferred by the facile loss of an OH group from the beta carbon of serine. In addition to the bare parent anions and various fragment anions, helium atoms are also observed attached to both the parent anion clusters and the dehydrogenated parent anion clusters. Finally, we present the first anion yield spectra of amino acid clusters from doped helium nanodroplets as a function of incident electron energy.
The Journal of chemical physics 06/2010; 132(21):214306. · 3.09 Impact Factor
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A Edtbauer,
P Sulzer,
A Mauracher,
C Mitterdorfer,
F Ferreira da Silva, S Denifl,
T D Märk,
M Probst,
Y Nunes,
P Limão-Vieira,
P Scheier
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ABSTRACT: Gas phase dissociative electron attachment (DEA) measurements to pentaerythritol tetranitrate (PETN) are performed in a crossed electron-molecular beam experiment at high-energy resolution and high sensitivity. DEA is operative at very low energies close to approximately 0 eV showing unique features corresponding to a variety of fragment anions being formed. There is no evidence of the parent anion formation. The fragmentation yields are also observed for higher electron energies and are operative via several resonant features in the range of 0-12 eV. In contrast to nitroaromatic compounds, PETN decays more rapidly upon electron attachment and preferentially low-mass anions are formed. The dominant fragment ion formed through DEA is assigned to the nitrogen trioxide NO(3)(-) and represents about 80% of the total anion yield. Further intense ion signals are due to NO(2)(-) (11%) and O(-) (2.5%). The significant instability of PETN after attachment of an electron with virtually no kinetic energy confers a highly explosive nature to this compound.
The Journal of chemical physics 04/2010; 132(13):134305. · 3.09 Impact Factor
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ABSTRACT: Electron attachment to formamide clusters in helium nanodroplets is reported for the first time. In contrast to the gas phase, parent anions are seen following low energy electron attachment to both the monomer and the small clusters. This is attributed to formation of dipole (or quadrupole) bound anions. In addition to the bare anions, the mass spectra also show the monomer and clusters with attached helium atoms. The affinity for attaching helium atoms strongly varies with cluster size; for example, the dimer anion is more than 10 times more likely to bind one or more helium atoms than the monomer. Possible binding sites for the helium atoms are discussed.
The Journal of Physical Chemistry A 02/2010; 114(4):1633-8. · 2.95 Impact Factor
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ABSTRACT: Dissociative electron attachment (DEA) to the DNA base adenine became of a considerable interest since the discovery that low energy electrons can effectively damage DNA below the ionization threshold. In order to understand the mechanisms of damaging the complex DNA structure, the knowledge about the dissociative electron attachment process to DNA building blocks has to be collected. Here we present a detailed study of the dissociative electron attachment to adenine with special emphasis upon the various fragmentation pathways. The dominant low energy resonances at about 1 eV are investigated by means of partially deuterated and methylated adenine compounds. Insight into the fragmentation process for DEA at higher electron energies is obtained by embedding adenine molecules into cold helium droplets.
Acta Physica Universitatis Comenianae. 01/2010; L-LI(1, 2):67-76.
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F Ferreira da Silva,
S Jaksch,
G Martins,
H M Dang,
M Dampc, S Denifl,
T D Märk,
P Limão-Vieira,
J Liu,
S Yang,
A M Ellis,
P Scheier
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ABSTRACT: The effect of incident electrons on acetic acid clusters is explored for the first time. The acetic acid clusters are formed inside liquid helium nanodroplets and both cationic and anionic products ejected into the gas phase are detected by mass spectrometry. The cation chemistry (induced by electron ionization at 100 eV) is dominated by production of protonated acetic acid (Ac) clusters, Ac(n)H(+), although some fragmentation is also observed. In the case of anion production (at 2.8 eV electron energy) there is a clear distinction between the monomer and the clusters. For the monomer the dominant product is the dehydrogenated species, [Ac-H](-), whereas for the clusters both the parent anion, Ac(n)(-), and the dehydrogenated species, [Ac(n)-H](-), have similar abundances. A particularly intriguing contrast between the monomer and cluster anions is that helium atoms are seen attached to the latter whereas no evidence of helium atom attachment is found for the monomer. This surprising observation is attributed to the formation of acyclic (head-to-tail) acetic acid clusters in helium nanodroplets, which have more favourable electronic properties for binding helium atoms. The acyclic clusters represent a local minimum on the potential energy surface and in the case of the dimer this is distinct from the cyclic isomer (the global minimum) identified in gas phase experiments.
Physical Chemistry Chemical Physics 12/2009; 11(48):11631-7. · 3.57 Impact Factor
