V. McKoy

National Research Council Canada, Ottawa, Ontario, Canada

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Publications (387)985.91 Total impact

  • [Show abstract] [Hide abstract]
    ABSTRACT: We report results of the application of a fully ab initio approach for simulating time-resolved molecular-frame photoelectron angular distributions around conical intersections in CS2. The technique employs wave packet densities obtained with the multiple spawning method in conjunction with geometry- and energy-dependent photoionization matrix elements. The robust agreement of these results with measured molecular-frame photoelectron angular distributions for CS2 demonstrates that this technique can successfully elucidate, and disentangle, the underlying nuclear and photoionization dynamics around conical intersections in polyatomic molecules.
    Physical Review Letters 03/2014; 112(11):113007. · 7.94 Impact Factor
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    ABSTRACT: We report differential and integral cross sections for excitation of the 3B1, 1B1, 3A2, 1A2, 3A1 and 1A1 states of H2O by 9–20 eV electrons. The measurements are taken by conventional differential electron energy loss spectroscopy techniques, while the calculations employ the Schwinger multichannel method within a ten-channel approximation. The new data are compared with previous experimental and theoretical results. The present measurements and calculations agree reasonably well both with each other and with prior theoretical efforts but show discrepancies with prior measurements. Reasons for those discrepancies are considered and discussed.
    Journal of Physics B Atomic Molecular and Optical Physics 05/2013; 46(12):125201. · 2.03 Impact Factor
  • Physical Review A 04/2013; 87(4). · 3.04 Impact Factor
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    ABSTRACT: We report measurements and first-principles calculations of the differential cross sections for elastic scattering of low-energy electrons by acetylene, C2H2, at collision energies from 1 to 100 eV, with an emphasis on energies near and below that of the π* shape resonance. The measurements cover angles from 5∘ to 130∘. We compare our results to previous experimental and theoretical values.
    Physical Review A 01/2013; 87(1). · 3.04 Impact Factor
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    ABSTRACT: We present absolute experimental cross sections for elastic scattering, vibrational excitation by electron impact and for dissociative electron attachment to 1,3-butadiyne, as well as calculations of the elastic cross sections.
    Journal of Physics Conference Series 11/2012; 388(5):2082-.
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    ABSTRACT: In order to understand and optimize processes occurring during the ignition of plasma and its consequences in post-discharge for an internal combustion engine, especially considering the spark plug, we have produced in this work some basic information necessary to modeling spark ignition in alcohol- fuelled engines. Total cross sections of electron scattering by methanol and ethanol molecules in the energy range from 60 to 500 eV are reported, using the linear transmission method based on the Beer-Lambert law to first approximation. Aditionally to that, measurements and calculations of differential cross sections for elastic low-energy (rotationally unresolved) electron scattering were also discussed, for impact energies of 1, 2, 5, 10, 15, 20, 30, 50, and 100 eV and for scattering angles of 5°-130°. The measurements were obtained using the relative flow method with an aperture source, and calculations using two different implementations of the Schwinger multichannel method, one that takes all electrons into account and is adapted for parallel computers, and another that uses pseudopotentials and considers only the valence electrons.
    Journal of Physics Conference Series 11/2012; 388(1):2014-.
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    ABSTRACT: New experimental and theoretical data are presented for excitation of the 3B1 and 1B1 states of water by low-energy (9-15 eV) electron impact.
    Journal of Physics Conference Series 11/2012; 388(5):2056-.
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    ABSTRACT: We report experimental and theoretical (Schwinger Multi-Channel method with polarization effects) differential cross-sections (DCS) for low energy elastic electron scattering from tetrahydrofuran. The data are for incident energies from 0.75 to 30eV and the experimental scattering angles range from 10^o to 130^o. Agreement between theory and experiment is very good across the range of this data. Comparisons with previous experiments is also very good in general. We will also report DCSs for vibrational excitation of this target for energies up to 15eV and similar scattering angles.
    10/2012;
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    ABSTRACT: Pyrazine (C4H4N2) is a model molecule for studying electron interactions with nucleases. Also pyrazine is an ideal target, due to its high symmetry (D2h), for theoreticians to investigate electron collisions with complex DNA/RNA bases. In this work we present absolute elastic differential cross sections and elastic excitation functions for scattering of low-energy electrons by pyrazine measured using a crossed electron-target beam apparatus at the Australian National University. A comparison is also made between pyrazine cross sections with previously measured pyrimidine and benzene cross sections. Since all those molecules are similar in structure and considered as analogues to nucleobases, we intend to discuss similarities and differences in electron scattering results between three molecules.
    10/2012;
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    ABSTRACT: We report experimental and theoretical (Schwinger Multi-Channel method, with polarization effects) for electron scattering from acetaldehyde. The incident energies range from 1eV to 30eV and scattering angles from 10 to 130^o. The experimental method used a moveable aperture source with the relative flow method. Comparisons between theory and experiment will be presented.
    10/2012;
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    ABSTRACT: We report measured and calculated differential cross-sections for inelastic scattering of low-energy electrons by water leading to excitation of the dissociative (1b1 → 4a1) 1, 3B1 states. The measurements were taken using conventional energy-loss spectroscopy at incident energies of 9, 10, 12, 15, and 20 eV for scattering angles from 10° to 130°. The calculations were carried out using the Schwinger multichannel method, with a Born-dipole correction applied in the singlet excitation channel. Integral excitation cross sections for the \tilde{a}\,^3B_1 and \tilde{A}\,^1B_1 states are also derived from the differential cross section results.
    Journal of Physics B Atomic Molecular and Optical Physics 10/2012; 45(20):1001-. · 2.03 Impact Factor
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    ABSTRACT: We explore the application of a quantum vibrational wavepacket dynamics formulation of femtosecond energy- and angle-resolved photoelectron spectroscopy to the study of nuclear and electron dynamics. The formulation incorporates geometry- and energy-dependent photoionization matrix elements and is well suited for studies of nonadiabatic dynamics where nuclear and electronic degrees of freedom are coupled. In this paper we explore two aspects of angle-resolved pump–probe photoelectron spectroscopy, using NO2 and NaI as examples. The first is a refinement of femtosecond photoelectron spectroscopy in which the time derivative of the velocity map image is seen to more readily characterize the nuclear wavepacket dynamics in non-adiabatic regions. The other is an analysis to extract the effects of electronic-state fluctuation from sub-femtosecond scale photoelectron dynamics. Within the framework of the Born–Huang (Oppenheimer) expansion where the total wavefunction is expressed in products of stationary-state electronic wavefunctions and time-dependent nuclear wavepackets, we seek to identify how the effect of electron dynamics might be manifested in principle.
    Journal of Physics B Atomic Molecular and Optical Physics 09/2012; 45(19):194006. · 2.03 Impact Factor
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    ABSTRACT: Cross sections for elastic scattering of low-energy electrons by tetrahydrofuran, a prototype for the furanose ring found in the backbone of DNA, have been measured and calculated over a wide energy range, with an emphasis on energies below 6 eV, where previous data are scarce. The measurements employ a thin-aperture version of the relative-flow method, while the calculations employ the Schwinger multichannel method with an extensive treatment of polarization effects. Comparisons with earlier results, both experimental and theoretical, are presented and discussed. A proper accounting for the strong permanent electric dipole of tetrahydrofuran is found to be essential to obtaining reliable cross sections, especially at energies below 5 eV.
    Physical Review A 05/2012; 85(5). · 3.04 Impact Factor
  • Carl Winstead, Vincent McKoy
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    ABSTRACT: We review applications of the Schwinger multichannel method to low-energy electron collisions with polyatomic molecules of biological interest. After briefly describing the method, its implementation, and its strengths and limitations, we turn to a discussion of specific molecular systems, with an emphasis on studies related to radiation damage to DNA mediated by secondary electrons. Throughout, we situate our results in the context of calculated and experimental data on electron scattering, dissociative attachment, and other relevant processes.
    Radiation Damage in Biomolecular Systems. 01/2012;
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    ABSTRACT: We present results of measurements and calculations of elastic electron scattering from pyrimidine in the energy range 3–50 eV. Absolute differential and integral elastic cross sections have been measured using a crossed electron-molecule beam spectrometer and the relative flow technique. The measured cross sections are compared with results of calculations using the well-known Schwinger variational technique and an independent-atom model. Agreement between the measured differential cross sections and the results of the Schwinger calculations is good at lower energies but less satisfactory at higher energies where inelastic channels that should be open are kept closed in the calculations.
    Physical Review A 12/2011; 84(6). · 3.04 Impact Factor
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    ABSTRACT: We report measured and calculated cross sections for elastic scattering of low-energy electrons by isopropanol (propan-2-ol). The experimental data were obtained using the relative flow technique with helium as the standard gas and a thin aperture as the collimating target gas source, which permits use of this method without the restrictions imposed by the relative flow pressure conditions on helium and the unknown gas. The differential cross sections were measured at energies of 1.5, 2, 3, 5, 6, 8, 10, 15, 20, and 30 eV and for scattering angles from 10∘ to 130∘. The cross sections were computed over the same energy range employing the Schwinger multichannel method in the static-exchange plus polarization approximation. Agreement between theory and experiment is very good. The present data are compared with previously calculated and measured results for n-propanol, the other isomer of C3H7OH. Although the integral and momentum transfer cross sections for the isomers are very similar, the differential cross sections show a strong isomeric effect: In contrast to the f-wave behavior seen in scattering by n-propanol, d-wave behavior is observed in the cross sections of isopropanol. These results corroborate our previous observations in electron collisions with isomers of C4H9OH.
    Physical Review A 10/2011; 84(4). · 3.04 Impact Factor
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    ABSTRACT: Pyrimidine (C4H4N2) is a model molecule for studying both electron and positron interactions with DNA/RNA bases, as three of the five nucleobases (cytosine, thymine, and uracil) are pyrimidine derivatives. In this work we present absolute elastic differential cross sections for scattering of low-energy electrons (3-50 eV) by pyrimidine measured using a crossed electron-target beam apparatus at the Australian National University. Cross sections calculated using the Schwinger variational technique and a screening-corrected form of the independent-atom method are provided for comparison with the measured results. We also present absolute total and differential cross sections, recently measured at the positron beamline facility at the Australian National University, for positron scattering from pyrimidine. The measurements are taken using a magnetically confined positron beam (1-200 eV) with typical energy resolution of 60 meV. These measured cross sections are used to discuss key features between electron and positron scattering processes.
    Journal of Physics Conference Series 06/2011; 388(5):5006P-.
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    ABSTRACT: We have previously shown how femtosecond angle- and energy-resolved photoelectron spectroscopy can be used to monitor quantum wavepacket bifurcation at an avoided crossing or conical intersection and also how a symmetry-allowed conical intersection can be effectively morphed into an avoided crossing by photo-induced symmetry breaking. The latter result suggests that varying the parameters of a laser to modify a conical intersection might control the rate of passage of wavepackets through such regions, providing a gating process for different chemical products. In this paper, we show with full quantum mechanical calculations that such optical control of conical intersections can actually be monitored in real time with femtosecond angle- and energy-resolved photoelectron spectroscopy. In turn, this suggests that one can optimally control the gating process at a conical intersection by monitoring the photoelectron velocity map images, which should provide far more efficient and rapid optimal control than measuring the ratio of products. To demonstrate the sensitivity of time-resolved photoelectron spectra for detecting the consequences of such optical control, as well as for monitoring how the wavepacket bifurcation is affected by the control, we report results for quantum wavepackets going through the region of the symmetry-allowed conical intersection between the first two (2)A' states of NO(2) that is transformed to an avoided crossing. Geometry- and energy-dependent photoionization matrix elements are explicitly incorporated in these studies. Time-resolved photoelectron angular distributions and photoelectron images are seen to systematically reflect the effects of the control pulse.
    Physical Chemistry Chemical Physics 02/2011; 13(19):8681-9. · 3.83 Impact Factor
  • Source
    M. Allan, C. Winstead, V. McKoy
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    ABSTRACT: We report measured and calculated differential elastic cross sections for collisions of low-energy electrons with diacetylene (1,3-butadiyne). A generally satisfactory agreement between theory and experiment has been found. The calculated cross sections provide interesting insight into the underlying resonant structure.
    Physical Review A 01/2011; 83(6):062703-062703. · 3.04 Impact Factor
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    ABSTRACT: We present differential, integral, and momentum-transfer cross sections for elastic scattering of low-energy electrons by three butanol isomers, isobutanol, t-butanol, and 2-butanol. Our results were calculated with the Schwinger multichannel method in the static-exchange plus polarization approximation for collision energies from 1 to 50 eV. The present results are compared with previous calculations and measurements for the remaining C{sub 4}H{sub 9}OH isomer, n-butanol [Khakoo et al., Phys. Rev. A 78, 062714 (2008)]. Distinctive behavior is observed in the differential cross sections at collision energies between 5 and 10 eV. In particular, whereas n-butanol exhibits an f-wave scattering pattern, the other isomers exhibit d-wave behavior. A similar pattern is found in the related alkanes when comparing straight-chain versus branched isomers. We discuss the possible connection of this behavior to shape resonances that influence the scattering.
    Physical Review A 12/2010; 82(6). · 3.04 Impact Factor

Publication Stats

5k Citations
985.91 Total Impact Points

Institutions

  • 2014
    • National Research Council Canada
      Ottawa, Ontario, Canada
  • 1967–2013
    • California Institute of Technology
      • • Arthur Amos Noyes Laboratory of Chemical Physics
      • • Division of Chemistry and Chemical Engineering
      • • Gates and Crellin Laboratories of Chemistry
      Pasadena, California, United States
  • 2000–2011
    • The University of Tokyo
      • College of Art and Science & Graduate School of Arts and Sciences
      Tokyo, Tokyo-to, Japan
  • 2010
    • California State University, Fullerton
      • Department of Physics
      Fullerton, California, United States
  • 2008
    • Tohoku University
      • Institute of Multidisciplinary Research for Advanced Materials (IMRAM)
      Sendai-shi, Miyagi-ken, Japan
    • Flinders University
      • School of Chemical and Physical Sciences
      Adelaide, South Australia, Australia
  • 2006
    • University of Campinas
      • Instituto de Física "Gleb Wataghin" (IFGW)
      Campinas, Estado de Sao Paulo, Brazil
  • 1994–2004
    • Louisiana State University
      • Department of Chemistry
      Baton Rouge, LA, United States
  • 1989
    • Lawrence Livermore National Laboratory
      Livermore, California, United States
  • 1986
    • Texas A&M University
      College Station, Texas, United States
  • 1981
    • Bielefeld University
      Bielefeld, North Rhine-Westphalia, Germany
  • 1977
    • University of Chicago
      • James Franck Institute
      Chicago, IL, United States