V. McKoy

California Institute of Technology, Pasadena, California, United States

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Publications (419)1007.46 Total impact

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    ABSTRACT: We measured differential cross sections for elastic (rotationally integrated) electron scattering on pyrimidine, both as a function of angle up to 180(∘) at electron energies of 1, 5, 10, and 20 eV and as a function of electron energy in the range 0.1-14 eV. The experimental results are compared to the results of the fixed-nuclei Schwinger variational and R-matrix theoretical methods, which reproduce satisfactorily the magnitudes and shapes of the experimental cross sections. The emphasis of the present work is on recording detailed excitation functions revealing resonances in the excitation process. Resonant structures are observed at 0.2, 0.7, and 4.35 eV and calculations for different symmetries confirm their assignment as the X̃(2)A2, Ã(2)B1, and B̃(2)B1 shape resonances. As a consequence of superposition of coherent resonant amplitudes with background scattering the B̃(2)B1 shape resonance appears as a peak, a dip, or a step function in the cross sections recorded as a function of energy at different scattering angles and this effect is satisfactorily reproduced by theory. The dip and peak contributions at different scattering angles partially compensate, making the resonance nearly invisible in the integral cross section. Vibrationally integrated cross sections were also measured at 1, 5, 10 and 20 eV and the question of whether the fixed-nuclei cross sections should be compared to vibrationally elastic or vibrationally integrated cross section is discussed.
    Full-text · Article · Jan 2016 · The Journal of Chemical Physics
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    ABSTRACT: We report theoretical as well as (normalized) experimental differential and integral cross sections for vibrationally elastic scattering of low-energy electrons from chloroethane, C2H5Cl, also known as ethyl chloride. The theoretical cross sections were computed using the Schwinger multichannel variational method in the single-channel approximation, with polarization effects included via virtual excitations. Cross section measurements were made at incident energies ranging from 1 to 30 eV and at scattering angles from &${10}^{\circ }$; to &${125}^{\circ }$;. We compare our data to previous results for C2H5Cl and for the related molecule chloromethane.
    No preview · Article · Oct 2015 · Journal of Physics B Atomic Molecular and Optical Physics
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    ABSTRACT: We report theoretical as well as (normalized) experimental differential and integral cross sections for vibrationally elastic scattering of low-energy electrons from chloromethane, CH3Cl, also known as methyl chloride. The theoretical cross sections were computed using the Schwinger multichannel variational method in the single-channel approximation, with polarization effects included via virtual excitations. Cross section measurements were made at incident energies ranging from 0.5 to 100 eV and at scattering angles from to 5° to 125°. We compare our data to earlier previous results for this molecule.
    No preview · Article · Oct 2015 · Journal of Physics B Atomic Molecular and Optical Physics
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    ABSTRACT: We report differential and integral cross sections for electronic excitation of methanol by low-energy electron impact. Cross sections were measured for the four lowest-lying excited states at incident electron energies from 9 to 20 eV and at scattering angles from 5° through 130°. The measured cross sections were normalized against previously reported elastic scattering data determined using the relative flow method. Corresponding cross-section calculations were carried out using the complex Kohn variational method within a 7-channel close-coupling scheme and using the Schwinger multichannel method within an 11-channel close-coupling scheme.
    No preview · Article · Jun 2015 · Journal of Physics B Atomic Molecular and Optical Physics
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    ABSTRACT: We describe a theoretical approach to molecular photoionization that includes first-order corrections to the dipole approximation. The theoretical formalism is presented and applied to photoionization of H2 over the 20- to 180-eV photon energy range. The angle-integrated cross section σ, the electric dipole anisotropy parameter βe, the molecular alignment anisotropy parameter βm, and the first-order nondipole asymmetry parameters γ and δ were calculated within the single-channel, static-exchange approximation. The calculated parameters are compared with previous measurements of σ and βm and the present measurements of βe and γ+3δ. The dipole and nondipole angular distribution parameters were determined simultaneously using an efficient, multiangle measurement technique. Good overall agreement is observed between the magnitudes and spectral variations of the calculated and measured parameters. The nondipole asymmetries of He 1s and Ne 2p photoelectrons were also measured in the course of this work.
    No preview · Article · May 2015 · Physical Review A
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    ABSTRACT: We report a combined experimental and theoretical investigation of dissociative electron attachment (DEA) to the nucleobase uracil. Using ion momentum imaging experiments employing a DEA reaction microscope we have measured 3-dimensional momentum distributions of specific anionic fragments following DEA to uracil by 6 eV electrons. From the measured anion fragment kinetic energy we determine the possible dissociation pathways and the total kinetic energy release. We employ electronic structure and electron scattering calculations to determine the probability for electron attachment in the molecular frame. Combining these calculations with the imaging measurements, we reveal several key features of the coupled electronic and nuclear dynamics of DEA.
    Full-text · Article · Oct 2014 · Journal of Physical Chemistry Letters
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    ABSTRACT: Normalized experimental differential and integral cross sections for vibrationally elastic scattering of low-energy electrons from isobutanol (C4H9OH) are presented. The differential cross sections are measured at incident energies from 1 to 100 eV and scattering angles from 5 degrees to 130 degrees. These cross sections are compared to earlier experimental and theoretical results for isobutanol and n-butanol, as well as to results for smaller alcohols and for alkanes. Further comparisons are made with calculated cross sections for isobutylamine (C4H9NH2) and for smaller amines, including ethylamine (C2H5NH2), dimethylamine (CH3NHCH3), the two C3H7NH2 isomers n-propylamine and isopropylamine, and ethylene diamine (NH2C2H4NH2). The calculated cross sections are obtained using the Schwinger multichannel method. The comparisons illuminate the role of molecular structure in determining the angular distribution of resonantly scattered electrons.
    Full-text · Article · Sep 2014 · Physical Review A
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    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 CS_2. 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 CS_2 demonstrates that this technique can successfully elucidate, and disentangle, the underlying nuclear and photoionization dynamics around conical intersections in polyatomic molecules.
    Full-text · Article · Mar 2014 · Physical Review Letters
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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.
    No preview · Article · May 2013 · Journal of Physics B Atomic Molecular and Optical Physics

  • No preview · Article · Apr 2013 · Physical Review A
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    ABSTRACT: We report results from a combined experimental and computational study of low-energy electron interactions with acetaldehyde in the gas phase. Differential cross sections for elastic electron scattering were measured at selected incident energies from 1 to 50 eV, while corresponding first-principles calculations were carried out up to 30 eV. Integral and momentum-transfer cross sections were derived from the angle-differential data. The role of resonances in the scattering is examined and comparison is made to previous results for acetaldehyde and for its analogs, formamide and formic acid.
    No preview · Article · Jan 2013 · Physical Review A
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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.
    Preview · Article · Nov 2012 · Journal of Physics Conference Series
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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.
    No preview · Article · Nov 2012 · Journal of Physics Conference Series
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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.
    No preview · Article · Nov 2012 · Journal of Physics Conference Series
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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 (1b_1 → 4ɑ_1) ^(1,3)B_1 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 ᾶ^3B_1 and Ã^1B_1 states are also derived from the differential cross section results.
    No preview · Article · Oct 2012
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    ABSTRACT: We report results from a combined experimental and computational study of low-energy electron interactions with acetaldehyde in the gas phase. Differential cross sections for elastic electron scattering were measured at selected incident energies from 1 to 50 eV, while corresponding first-principles calculations were carried out up to 30 eV. Integral and momentum-transfer cross sections were derived from the angle-differential data. The role of resonances in the scattering is examined and comparison is made to previous results for acetaldehyde and for its analogs, formamide and formic acid.
    No preview · Article · Oct 2012
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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.
    No preview · Article · Oct 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.
    No preview · Article · Oct 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.
    No preview · Article · Oct 2012 · Journal of Physics B Atomic Molecular and Optical Physics
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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.
    No preview · Article · Sep 2012 · Journal of Physics B Atomic Molecular and Optical Physics

Publication Stats

9k Citations
1,007.46 Total Impact Points

Institutions

  • 1967-2015
    • 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
  • 2014
    • National Research Council Canada
      Ottawa, Ontario, Canada
  • 2010-2013
    • California State University, Fullerton
      • Department of Physics
      Fullerton, California, United States
  • 2005
    • Universidade Federal do Paraná
      • Department of Physics
      Curityba, Paraná, Brazil
  • 2000
    • University of Minnesota Duluth
      • Department of Chemistry and Biochemistry
      Duluth, Minnesota, United States
  • 1994-1995
    • Louisiana State University
      • Department of Chemistry
      Baton Rouge, Louisiana, United States
  • 1987
    • NASA
      Вашингтон, West Virginia, United States
  • 1981
    • Bielefeld University
      Bielefeld, North Rhine-Westphalia, Germany
    • Pasadena City College
      Pasadena, Texas, United States