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S Feil,
P Sulzer,
A Mauracher,
M Beikircher,
N Wendt,
A Aleem,
S Denifl,
F Zappa,
S Matt-Leubner,
A Bacher, S Matejcik,
M Probst,
P Scheier,
T D Märk
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ABSTRACT: In order to provide quantitative information on electron collision processes involving various plasma constituents (in particular hydrocarbons) and to elucidate the properties of cations and anions produced we have carried out the past years a series of studies with a variety of techniques in our laboratory in Innsbruck. In the present review we will present some recent results on electron impact ionization and attachment in order to illustrate recent progress in this field in particular concerning the production of energetic fragment cations for hydrocarbons and differences in the attachment of isomers for nitro-organics. Using a Nier type electron impact ion source in combination with a double focusing two sector field mass spectrometer, partial cross sections for electron impact ionization of acetylene, propene and other hydrocarbons have been measured for electron energies up to 1000 eV. Discrimination factors for ions have been determined using the deflection field method in combination with a three-dimensional ion trajectory simulation of ions produced in the ion source. Analysis of the ion yield curves obtained by scanning the deflectors allows the assignment of ions with the same mass-to-charge ratio to specific production channels on the basis of their different kinetic energy distributions. This analysis also allows to determine, besides kinetic energy distributions of fragment ions, partial cross sections differential in kinetic energy. Moreover charge separation reactions (for instance in case of acetylene the Coulomb explosion of the doubly-charged parent ions C2H2++ into the fragment ions C2H+ and H+) are investigated by means of a number of metastable mass spectrometry methods and the associated mean kinetic energy release is deduced. Free electron attachment to the three different isomers of mono-nitrotoluene molecules in the gas phase is studied using two different crossed electron-molecule beams technique. In contrast to previous studies for a large number of negative ions in nitro-organic compounds 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 NO2− remarkable differences for the three isomers are observed. For almost all fragment anion efficiency curves the 2-nitrotoluene exhibits pronounced differences compared to the two other isomers. In contrast, 3- and 4-nitrotoluene disagree only slightly in a few fragment anions from each other.
Journal of Physics Conference Series 11/2007; 86(1):012003.
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ABSTRACT: The dissociative electron attachment to the gas phase nucleobase adenine is studied using two different experiments. A double focusing sector field mass spectrometer is utilized for measurements requiring high mass resolution, high sensitivity, and relative ion yields for all the fragment anions and a hemispherical electron monochromator instrument for high electron energy resolution. The negative ion mass spectra are discussed at two different electron energies of 2 and 6 eV. In contrast to previous gas phase studies a number of new negative ions are discovered in the mass spectra. The ion efficiency curves for the negative ions of adenine are measured for the electron energy range from about 0 to 15 eV with an electron energy resolution of about 100 meV. The total anion yield derived via the summation of all measured fragment anions is compared with the total cross section for negative ion formation measured recently without mass spectrometry. For adenine the shape of the two cross section curves agrees well, taking into account the different electron energy resolutions; however, for thymine some peculiar differences are observed.
The Journal of Chemical Physics 09/2006; 125(8):084304. · 3.33 Impact Factor
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S Denifl,
S Ptasinska,
G Hanel,
B Gstir,
P Scheier,
M Probst,
B Farizon,
M Farizon, S Matejcik,
E Illenberger,
T D Märk
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ABSTRACT: Electron attachment (EA) to uracil (U), thymine (T) and cytosine (C) was studied in the electron energy range from about 0 to 12 eV using a high resolution crossed electron/molecule beams technique. The dominant negative ions formed via EA reactions to U, T and C are (U - H)-, (T - H)- and (C - H)-. The respective partial EA cross sections could be determined yielding peak values of σ(1.0 eV) = 3 × 10-20 m2, σ(1.05 eV) = 1.2 × 10-19 m2, σ(1.54 eV) = 2.3 × 10-20 m2, respectively. Based on (i) a comparison of the resonance positions for the different bases and on (ii) high level ab initio calculations we can assign certain resonances to the site specific loss of hydrogen during the EA reaction. At higher electron energies, in the range between about 3 and 12 eV we observe further product anions (e.g., for U the ions CN-, OCN- and C3H2NO-), however, with significantly lower cross section values.
Physica Scripta 07/2006; 2004(T110):252. · 1.20 Impact Factor
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G. Hanel,
B. Gstir,
S. Denifl,
G. Denifl,
D. Muigg,
T. Fiegele,
M. Rümmele,
W. Sailer,
A. Pelc,
N. Mason,
E. Illenberger, S. Matejcik,
F. Hagelberg,
K. Becker,
A. Stamatovic,
M. Probst,
P. Scheier,
T.D. Märk
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ABSTRACT: Ionisation and attachment by electrons are two of the most fundamental inelastic electron collision processes. Electron-impact
ionisation/attachment processes are also important in many practical applications such as low-temperature plasma processing,
fusion edge plasmas, planetary atmospheres, radiation chemistry and chemical analysis. Considerable progress in the experimental
and theoretical description of electron-driven ionisation and attachment processes involving atomic and molecular targets
has been achieved in the past decade, for instance concerning the quantitative determination of total and partial electron
impact ionisation cross sections. Nevertheless, with respect to information about the finer details of this interaction, which
has to come primarily from experimental studies, little is known due to the fact that experiments require the availability
of electron beams of high quality in terms of electron energy resolution and accuracy.
The very recent development, refinement and application of new experimental techniques in our laboratories (novel types of
molecular beam sources and high resolution electron beam and mass spectrometry techniques, e. g., HEM-QM and TEM-QM allowing
us to achieve in both routinely electron energy resolutions of about 30 to 50 meV at electron currents in the nA range, which
are still high enough to carry out statistically relevant measurements for systems with low interaction cross sections) made
this the ideal time for carrying out a coordinated series of experiments planned to attack the many open questions in this
field. In this review we will first discuss the experimental set-up and techniques and then present some prototypical examples
including (i) the determination of appearance energies and Wannier exponents for multiple ionisation of rare gases, (ii) isotope
effects in the electron impact ionisation of H2/D2 and H2O/D2O, (iii) appearance energy, binding energy and structure of the ozone dimer and finally (iv) vibrational structure in the
dissociative electron attachment to formic acid.
01/2006: pages 149-177;
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P. Cicman,
K. Gluch,
A. Pelc,
W. Sailer,
S. Matt-Leubner,
P. Scheier, S. Matejcik,
P. Lukac,
W. D. Robertson,
R. N. Compton,
T. D. Märk
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ABSTRACT: Electron impact ionization of the chlorodifluoromethane molecule is studied using crossed beams of high-resolution electrons and an effusive molecular beam of CHF2Cl. Ionization energies (IEs) for many positive ions from CHF2Cl (CHF2Cl+,CF2Cl+,CHFCl+,CFCl+,CHF2+,CF2+,HCl+,Cl+,CF+,CH+,F+ C+) are determined from a careful examination of the threshold behavior of the ionization cross sections. Reaction pathways for the dissociative ionization products are suggested using known thermodynamic quantities. Surprisingly, it is observed that the ionization threshold for the parent positive ion IE(CHF2Cl+/CHF2Cl) = 12.50(±0.05) eV lies above that for the fragment ions CHF2+, CHFCl+, and CF+ [IE(CHF2+/CHF2Cl) = 12.24(±0.03) eV, IE(CHFCl+/CHF2Cl) = 12.3(±0.05) eV, and IE(CF+/CHF2Cl) = 11.5(±0.1) eV]. Experiments using a three sector field BEE mass spectrometer provide evidence for the existence of two states of the parent CHF2Cl+ ion, one exhibiting a short lifetime of about 2 μs and another parent ion state which appears to be stable on the time scale of the experiment (>20 μs) with an IE of 12.50 eV. © 2003 American Institute of Physics.
The Journal of Chemical Physics 12/2003; 119(22):11704-11711. · 3.33 Impact Factor
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ABSTRACT: Electron attachment (EA) and dissociative electron attachment (DEA) to 5-chloro uracil (5-ClU) was studied in the gas phase using a crossed electron/molecule beams technique. Besides production of a parent anion via a zero energy resonance, ion yields of nine different negative ions were observed in the electron energy range from about 0 to 14 eV. In the electron energy range from about zero to 5 eV, the formation of a transient negative ion was induced by electron attachment to the π∗ resonances located at about 0.24, 1.5, and 3.6 eV leading subsequently by unimolecular decay to various negative fragment ions. Absolute partial cross sections for EA and DEA to 5-ClU were obtained from the measured ion yields using a simple calibrating method. The dominant negative ion observed in the present experiment was (C4H2N2O2)− (corresponding to 5-ClU minus HCl) with a mass to charge ratio of 110, followed by Cl− ion (mass to charge ratios 35 and 37), the partial cross sections being σ(0.23 eV) = 5×10−18 m2 and σ(0.23 eV) = 3×10−18 m2, respectively. The parent anion produced (5-ClU)− has only a cross section value of σ(0 eV) = 3×10−20 m2. The energetic thresholds for the formation of particular negative ions from 5-ClU in the gas phase were calculated at the G2(MP2) level of theory and compared with the experimental results. On the basis of these calculations structure and relative stability of some of the fragment ions was predicted. In the case of the two most abundant ions, their formation was observed well below the calculated electron energy threshold. The formation of these negative ions below the thermodynamic threshold is explained in terms of the high vibrational energy of the 5-ClU prior to the EA process. © 2003 American Institute of Physics.
The Journal of Chemical Physics 02/2003; 118(9):4107-4114. · 3.33 Impact Factor
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Nuclear Instruments and Methods 01/2003; 205:413.
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ABSTRACT: We determined appearance energy (AE) values AE(Xn+/X) for the formation of singly (He+) and doubly charged (He2+) He ions and multiply charged Kr ions Krn+ up to n = 6 following electron impact on He and Kr atoms using a high-resolution electron impact ionization mass spectrometer. The data analysis employs an iterative, non-linear least-squares fitting routine, the Marquart–Levenberg algorithm, in conjunction with either a 2-function or a 3-function fit based on a power threshold law. This allows us to extract the relevant AEs and also the corresponding exponents for a Wannier-type power law from the measured near-threshold data. The values of the AEs determined in this work are compared with other available experimental and with spectroscopic AE values and the extracted exponents p are compared with other available experimental data and with the predictions of the various Wannier-type power law models. One observation is particularly noteworthy, namely the fact that none of the available experimental data seem to support the large values of 'p' predicted by the Wannier–Geltman and the generalized Wannier law for n > 3.
Journal of Physics B Atomic Molecular and Optical Physics 11/2002; 35(22):4685. · 1.88 Impact Factor
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B Gstir,
S Denifl,
G Hanel,
M Rümmele,
T Fiegele,
P Cicman,
M Stano, S Matejcik,
P Scheier,
K Becker,
A Stamatovic,
T D Märk
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ABSTRACT: We report the results of the experimental determination of the appearance energy values AE(Xn + /X) for the formation of multiply charged Ne, Ar and Xe ions up to n = 4 (Ne), n = 6 (Ar) and n = 8 (Xe) following electron impact on Ne, Ar and Xe atoms using a dedicated high-resolution electron impact ionization mass spectrometer. The data analysis uses the Marquart-Levenberg algorithm, which is an iterative, nonlinear least-squares-fitting routine, in conjunction with either a two-function or a three-function fit based on a power threshold law. This allows us to extract the relevant AEs and corresponding exponents for a Wannier-type power law from the measured near-threshold data. The values of the AEs determined in this work are compared with other available experimental and spectroscopic values of the AEs and the extracted exponents are compared with other available experimental data and with the predictions of the various Wannier-type power law models.
Journal of Physics B Atomic Molecular and Optical Physics 06/2002; 35(13):2993. · 1.88 Impact Factor
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A. Kendl,
K Becker,
S Denifl,
O Echt,
N. Endstrasser,
S. Feil,
B. Farizon,
L. Feketeova,
V Grill,
Z. Herman,
T D Märk, S. Matejcik,
S Matt-Leubner,
M Probst,
P Scheier,
W. Schustereder,
P Sulzer,
M. Winkler,
F Zappa,
B. Rasul
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ABSTRACT: The compatibility of reactor grade plasmas with plasma facing materials at the first wall is one of the present challenges in fusion research. In order to understand and elucidate the role of radiative and collisional processes in the edge plasma region, it is essential to have available detailed and quantitative knowledge on the corresponding elementary reactions proceeding in the volume before and at the wall. Surface processes involving hydrocarbons and their ions, which are probable vacuum contaminants in plasma devices, and the role of hydrocarbon chemistry and transport in divertor plasmas are some of the key components in modelling predictions for ITER based on atomic and molecular data. Improved data sets for hydrocarbons and detailed and accurate knowledge of the cross sections of the relevant plasma chemical volume and wall processes are needed. In this context we have carried out: electron (proton and helium) impact excitation/ionization reactions with possible plasma edge atoms, molecules and ions; determination of relevant differential, partial and total crosssections and reaction rate coefficients for electron-molecule and ion-surface collisions; investigation of the temperature dependence of ionization energies and ionization cross sections and electron attachment cross sections; and calculations of inelastic interactions between electrons and atoms, molecules and molecular ions. The reported work supports the provision of essential data commonly needed for diagnostics and modelling across the fusion programme.
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ABSTRACT: Electron attachment (EA) to the DNA bases thymine (T) and cytosine (C) was studied in the electron energy range from about 0 to 9 eV using a crossed electron/molecule beams technique. As uracil the dominant negative ions formed via EA reactions to T and C are (T − H)− (corresponding to T minus H, with a mass to charge ratio of 125) and (C − H)− (corresponding to C minus H with a mass to charge ratio of 110). The respective partial EA cross-sections for (T − H)− and (C − H)− could be determined yielding peak values of σ (1.05 eV)=1.2 × 10−19 m2, σ (1.54 eV)=2.3 × 10−20 m2, respectively.
Chemical Physics Letters.
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ABSTRACT: Using a high resolution electron energy monochromator the dissociative electron attachment (DEA) to the gas phase l-alanine is studied by means of the mass spectrometric detection of the product anions. Alanine and the previously studied amino acid glycine exhibit several common features due to the possibility of electron attachment to the unoccupied π* orbital of the –COOH group. The largest DEA cross-section of about 1.5 × 10−20 m2 is observed for the production of the (A − H)− ion at the electron energy of 1.27 eV. This ion is the major reaction product at electron energies below 5 eV. At higher incident electron energies several smaller fragment anions are formed via core excited resonances at about 5.5 and 9.0 eV.
Chemical Physics Letters.
