Ilya Fabrikant PublicationsCopyright (c) 2017 University of Nebraska - Lincoln All rights reserved.
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Recent documents in Ilya Fabrikant Publicationsen-usWed, 05 Apr 2017 01:43:54 PDT3600Near-threshold behavior of positronium-antiproton scattering
http://digitalcommons.unl.edu/physicsfabrikant/10
http://digitalcommons.unl.edu/physicsfabrikant/10Tue, 04 Apr 2017 14:16:28 PDT
Using the convergent close-coupling theory we study the threshold behavior of cross sections for positronium (Ps) of energy E scattering on antiprotons. In the case of Ps(1s) elastic scattering, simple power laws are observed for all partial waves studied. The partial-wave summed cross section is nearly constant, and dominates the antihydrogen formation cross section at all considered energies, even though the latter is exothermic and behaves as 1/E^{1/2}. For Ps(2s), oscillations spanning orders of magnitude on top of the 1/E behavior are found in the elastic and quasielastic cross sections. The antihydrogen formation is influenced by dipole-supported resonances below the threshold of inelastic processes. Resonance energies form a geometric progression relative to the threshold. The exothermic antihydrogen formation cross sections behave as 1/E at low energies, but are oscillation free. We demonstrate that all these rich features are reproduced by the threshold theory developed by Gailitis
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Ilya I. Fabrikant et al.Electron attachment to the interhalogen compounds ClF, ICl, and IBr
http://digitalcommons.unl.edu/physicsfabrikant/9
http://digitalcommons.unl.edu/physicsfabrikant/9Tue, 04 Apr 2017 14:13:25 PDT
Thermal electron attachment rate coefficients for three interhalogen compounds (C1F, ICl, IBr) have been measured from 300 to 900 K at pressures of 1–2 Torr using a flowing afterglow–Langmuir probe apparatus. ClF attaches somewhat inefficiently (k = 7.5×10−9 cm3 s−1) at 300 K, with the rate coefficient rising to 1.7×10^{−}^{8} cm^{3} s^{−}^{1} at 700 K. At higher temperatures the apparent rate coefficient falls steeply; however, this is interpreted as an artifact due to decomposition on the walls of the inlet line. ICl attaches with even lower efficiency (k = 9.5×10^{−}^{10} cm^{3} s^{−}^{1} at 300 K) and a less steep increase with temperature. Attachment to IBr is too slow to confidently measure with the present experiment, with an upper limit on the rate coefficient of 10^{−}^{10} cm^{3} s^{−}^{1} from 300 to 600 K. Both ClF and ICl attach dissociatively to yield Cl−, likely exclusively, though F− or I− may be produced with limits of <2% and <5%, respectively. The ClF attachment was further explored through ab initio calculation of the ClF and ClF− potential energy curves and R-matrix calculations of the resonance parameters which were used then for calculations of the dissociative attachment cross sections and rate coefficients. While the magnitude of the attachment rate coefficient for ClF is similar to those for both Cl_{2} and F_{2}, the calculated cross sections show qualitatively different threshold behavior due to the s-wave contribution allowed by the lack of inversion symmetry. The v = 1 and 2 vibrational modes of ClF attach about three to four times faster than v = 0 and 3 at energies lower than ∼0.2 eV. The calculated rate coefficients are in good agreement with the experiment at 300 K and increase moderately less steeply with temperature.
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Justin P. Wiens et al.Dissociative electron attachment to halogen molecules: Angular distributions and nonlocal effects
http://digitalcommons.unl.edu/physicsfabrikant/8
http://digitalcommons.unl.edu/physicsfabrikant/8Tue, 04 Apr 2017 14:07:24 PDT
We study dissociative electron attachment (DEA) to the ClF and F_{2} molecules. We formulate a method for calculation of partial resonance widths and calculate the angular distributions of the products in the ClF case using the local and nonlocal versions of the complex potential theory of DEA. They show the dominance of the p wave except in a narrow energy region close to zero energy. Comparison of the local and nonlocal DEA cross sections show that the former are smaller than the latter by a factor of 2 in the energy region important for calculation of thermal rate coefficients. This result is confirmed by comparison of the local and nonlocal calculations for F_{2}. Only at low energies below 30 meV the local cross sections exceed nonlocal due to the 1/E divergence of the local results. On the other hand, the thermal rate coefficients generated from the local cross sections agree better with experiment than those calculated from the nonlocal cross sections. The most likely reason for this disagreement is the overestimated resonance width in the region of internuclear distances close to the point of crossing between the neutral and anion potential-energy curves.
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Ilya I. FabrikantDissociative electron attachment and vibrational excitation of CF3Cl: Effect of two vibrational modes revisited
http://digitalcommons.unl.edu/physicsfabrikant/7
http://digitalcommons.unl.edu/physicsfabrikant/7Fri, 04 May 2012 14:40:27 PDT
We present a study of dissociative electron attachment and vibrational excitation processes in electron collisions with the CF_{3}Cl molecule. The calculations are based on the two-dimensional nuclear dynamics including the C-Cl symmetric stretch coordinate and the CF_{3} symmetric deformation (umbrella) coordinate. The complex potential energy surfaces are calculated using the ab initio R-matrix method. The results for dissociative attachment and vibrational excitation of the umbrella mode agree quite well with experiment whereas the cross section for excitation of the C-Cl symmetric stretch vibrations is about a factor-of-three too low in comparison with experimental data.
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Michal Tarana et al.Effects of two vibrational modes in the dissociative electron attachment to CF3Cl
http://digitalcommons.unl.edu/physicsfabrikant/6
http://digitalcommons.unl.edu/physicsfabrikant/6Mon, 07 Dec 2009 13:30:45 PST
We present a study of multimode effects in dissociative electron attachment to CF_{3}Cl molecules using a time-independent version of the local complex potential theory. Symmetric stretch C-Cl vibrations v_{3} and symmetric deformation (or so-called umbrella) vibrations v_{2} are included. The neutral and anion potential energy surfaces are calculated using the second-order Møller-Plesset perturbation theory with an empirical adjustment of the vertical attachment energy. The final-state vibrational distribution in the CF_{3}(v_{2}) fragment is dominated by the v_{2}=2 state. We also find an increase in the total cross section as compared with the one-dimensional calculations. This is explained by an increase in the anion survival probability.
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Michal Tarana et al.Highly resolved absolute cross-sections for dissociative electron attachment to SF5CF3
http://digitalcommons.unl.edu/physicsfabrikant/5
http://digitalcommons.unl.edu/physicsfabrikant/5Mon, 07 Dec 2009 13:25:13 PST
Using two complementary experimental methods, we have measured partial (mass-resolved) cross-sections for dissociative electron attachment to the molecule trifluoromethyl sulfurpentafluoride (SF_{5}CF_{3}) at the gas temperature T_{G} = 300K over a broad range of electron energies (E = 0.001–12 eV).
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K. Graupner et al.On the Validity of the Arrhenius Equation for Electron Attachment Rate Coefficients
http://digitalcommons.unl.edu/physicsfabrikant/4
http://digitalcommons.unl.edu/physicsfabrikant/4Wed, 08 Apr 2009 12:03:07 PDT
The validity of the Arrhenius equation for dissociative electron attachment rate coefficients is investigated. A general analysis allows us to obtain estimates of the upper temperature bound for the range of validity of the Arrhenius equation in the endothermic case and both lower and upper bounds in the exothermic case with a reaction barrier. The results of the general discussion are illustrated by numerical examples whereby the rate coefficient, as a function of temperature for dissociative electron attachment, is calculated using the resonance R-matrix theory. In the endothermic case, the activation energy in the Arrhenius equation is close to the threshold energy, whereas in the case of exothermic reactions with an intermediate barrier, the activation energy is found to be substantially lower than the barrier height.
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Ilya I. Fabrikant et al.Dissociative Electron Attachment on Surfaces and in Bulk Media
http://digitalcommons.unl.edu/physicsfabrikant/3
http://digitalcommons.unl.edu/physicsfabrikant/3Wed, 08 Apr 2009 12:03:07 PDT
A theory of dissociative electron attachment to molecules condensed at surfaces and embedded within bulk media is developed. The theory of low-energy electron diffraction is used to obtain the width for electron-molecule resonance scattering in the condensed phase from the width for the gas phase. It is then employed for the calculation of dissociative attachment in the framework of the nonlocal complex potential theory. Specific calculations using the effective mass approximation are carried out for electron attachment to CH_{3}Cl and CF_{3}Cl molecules physisorbed on the surface of a Kr film. The role of image states and image-potential resonances is analyzed. The results show an increase by several orders of magnitude in the cross section for physisorbed molecules as compared with gas-phase molecules. This is in general agreement with the measured cross sections. However, the position of the peak in the cross section for CH_{3}Cl is significantly shifted towards higher energies as compared to experiment [K. Nagesha et al., J. Chem. Phys. 114, 4934 (2001)], and the magnitude of the calculated cross section for CF_{3}Cl at the surface is significantly higher than the measured value. Possible reasons for disagreements are analyzed.
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Ilya I. FabrikantElectron Attachment to Cl<sub>2</sub> from 300 to 1100 K: Experiment and Theory
http://digitalcommons.unl.edu/physicsfabrikant/2
http://digitalcommons.unl.edu/physicsfabrikant/2Wed, 08 Apr 2009 12:03:06 PDT
Rate constants for dissociative electron attachment to Cl_{2} have been measured from 300 to 1100 K in a high-temperature flowing-afterglow Langmuir-probe apparatus. R-matrix calculations have been carried out which compare well with the present measurements with possible deviation at the highest temperatures. The attachment rate constants do not show Arrhenius behavior. The temperature dependence of the calculated rate constants for successive vibrational levels provides insight as to this behavior. While the lowest vibrational level of Cl_{2} dominates attachment at low temperatures, the rate constant is not flat with temperature because of the p-wave character of the attachment process. The non-Arrhenius behavior is due to a conflict between the increase in attachment cross section with vibrational level (temperature) and the decline in the cross section with electron energy above 50 meV.
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Jeffrey Friedman et al.Observation of <i>p</i>-Wave Threshold Behavior in Electron Attachment to F<sub>2</sub> Molecules
http://digitalcommons.unl.edu/physicsfabrikant/1
http://digitalcommons.unl.edu/physicsfabrikant/1Wed, 08 Apr 2009 12:03:06 PDT
Using the high resolution laser photoelectron attachment method, we demonstrate that the cross section for F^{-} formation due to electron capture by F^{2} (X^{1}Σ_{g}^{+}) molecules at very low energies exhibits p-wave threshold behavior. This finding confirms the theoretical expectation that low-energy attachment to F_{2} proceeds through the F_{2}^{-}(^{2} Σ_{u}^{+}) p-wave shape resonance in contrast with previous experimental claims for s-wave threshold behavior.
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M. Braun et al.