Carl Winstead

Low-energy electron collisions with gas-phase uracil

We have studied gas-phase collisions between slow electrons and uracil molecules with a view to understanding the resonance structure of the scattering cross section. Our symmetry-resolved results for elastic scattering, computed in the fixed-nuclei, static-exchange and static-exchange-plus-polarization approximations, provide locations for the expected pi* shape resonances and indicate the possible presence of a low-energy sigma* resonance as well. Electron-impact excitation calculations were carried out for low-lying triplet and singlet excitation channels and yield a very large singlet cross section. We discuss the connection between the resonances found in our elastic cross section and features observed in dissociative attachment.

Low-energy electron scattering by deoxyribose and related molecules

We apply first-principles computational methods to study elastic scattering of low-energy electrons by 2-deoxyribose and 2-deoxyribose monophosphate, which are of interest as components of the DNA backbone, and to tetrahydrofuran (THF), which has been studied as a deoxyribose analog. To investigate the dependence of the scattering process on the molecular conformation, we examine Cs and C2 conformers of THF as well as the planar C(2v) geometry imposed in earlier calculations. There is little difference between the elastic cross sections determined at the nonplanar geometries, but there are noticeable differences between those results and the cross sections computed using the planar ring. By comparing results for tetrahydrofuran obtained with and without inclusion of polarization effects, we obtain energy shifts that are applied to the computed resonance positions for deoxyribose and deoxyribose monophosphate.

Interaction of low-energy electrons with the pyrimidine bases and nucleosides of DNA

We report computed cross sections for the elastic scattering of slow electrons by the pyrimidine bases of DNA, thymine and cytosine, and by the associated nucleosides, deoxythymidine and deoxycytidine. For the isolated bases, we carried out calculations both with and without the inclusion of polarization effects. For the nucleosides, we neglect polarization effects but estimate their influence on resonance positions by comparison with the results for the corresponding bases. Where possible, we compare our results with experiment and previous calculations.

Interaction of low-energy electrons with the purine bases, nucleosides, and nucleotides of DNA

The authors report results from computational studies of the interaction of low-energy electrons with the purine bases of DNA, adenine and guanine, as well as with the associated nucleosides, deoxyadenosine and deoxyguanosine, and the nucleotide deoxyadenosine monophosphate. Their calculations focus on the characterization of the pi* shape resonances associated with the bases and also provide general information on the scattering of slow electrons by these targets. Results are obtained for adenine and guanine both with and without inclusion of polarization effects, and the resonance energy shifts observed due to polarization are used to predict pi* resonance energies in associated nucleosides and nucleotides, for which static-exchange calculations were carried out. They observe slight shifts between the resonance energies in the isolated bases and those in the nucleosides.

Studies of electron collisions with polyatomic molecules using distributed‐memory parallel computers

Elastic electron scattering cross sections from 5–30 eV are reported for the molecules C2H4, C2H6, C3H8, Si2H6, and GeH4, obtained using an implementation of the Schwinger multichannel method for distributed‐memory parallel computer architectures. These results, obtained within the static‐exchange approximation, are in generally good agreement with the available experimental data. These calculations demonstrate the potential of highly parallel computation in the study of collisions between low‐energy electrons and polyatomic gases. The computational methodology discussed is also directly applicable to the calculation of elastic cross sections at higher levels of approximation (target polarization) and of electronic excitation cross sections.

Electron transport properties and collision cross sections in C2F4

We have measured the electron drift velocity, longitudinal diffusion coefficient, and ionization coefficient in tetrafluoroethene (C_2F_4). Using these data and the results of ab initio calculations of the elastic, momentum-transfer, and neutral-excitation cross sections, along with measurements of the partial ionization cross sections, we have performed a swarm analysis in order to construct a self-consistent set of electron impact cross sections for C_2F_4. The swarm analysis consists of solutions to Boltzmann’s equation for electrons in C_2F_4 for values of E/N⩽500Td and direct Monte Carlo simulation of electron transport in C_2F_4 for 500Td⩽E/N⩽2000Td. We present an analysis and discussion of the sensitivity of cross sections derived from swarm data to uncertainties in the electron transport measurements. We also discuss the failure of the two-term spherical harmonic solution to Boltzmann’s equation for E/N>500Td, which necessitated the use of Monte Carlo simulations for high values of E/N.

Highly parallel computational techniques for electron-molecule collisions

Though most of the current knowledge of electron–molecule collisions derives from experiments, accurate measurements of collision cross sections are quite difficult, especially for inelastic processes; few groups worldwide have undertaken this challenging work. Demand for cross section data already exceeds supply and the list of “critical” but absent data grows longer daily. Moreover, the species of interest include not only stable molecules, but also those radicals and ions whose populations within the plasma may be significant and experiments will be all the more difficult for such transient species. Most recent theoretical studies of electron–molecule collisions have relied on variational approximations to the scattering amplitude or to some closely related quantity, thereby avoiding direct numerical solution of Schrodingers equation. However, the studies of many-electron systems remain numerically intensive despite the choice of an efficient theoretical approach and the work discussed in the chapter depends on exploiting the prodigious advances in computational power that have resulted from the development of massively parallel processors.

Elastic scattering of low-energy electrons by benzene

We present elastic cross sections obtained from ab initio calculations for low-energy electron scattering by benzene, C6H6. The calculations employed the Schwinger multichannel method as implemented for parallel computers within both the static-exchange and static-exchange-polarization approximations. We compare our results with other theoretical calculations and with available experimental data. In general, agreement is good.

Low-energy electron collisions with tetrafluoroethene, C2F4

We report calculated cross sections for elastic and inelastic collisions of low-energy electrons with tetrafluoroethene, C_2F_4. The elastic cross section shows a number of resonance features, which we classify according to symmetry and analyze in relation to available experimental data. Electron-impact excitation cross sections are obtained for the 1^3B_(1u)(T) and 1^1B_(1u)(V) states arising from the π→π* transition, as well as for eight other low-lying excited states arising from excitations out of the highest occupied molecular orbital. As expected, the T and V states make the largest individual contributions to electron-impact excitation at low energies; however, the other states are shown to contribute significantly to the total excitation cross section at impact energies from 10 to 25 eV.

Electron collisions with octafluorocyclobutane, c-C4F8

We present calculated cross sections for elastic and inelastic collisions of low-energy electrons with octafluorocyclobutane, c-C_4F_8. The integral elastic cross section displays a rich resonance structure, which we analyze in terms of temporary trapping in virtual valence orbitals. The differential elastic cross sections compare well with recent measurements at energies where the approximations used in the calculations are expected to be valid. Integral and differential cross sections for electron-impact excitation of the lowest singlet and triplet excited states were obtained. We relate the small magnitude of the inelastic integral cross sections and the unusual form of the inelastic differential cross sections to the symmetries of the electronic states involved in the transition.