Low-energy elastic electron interactions with pyrimidine

We present results of measurements and calculations of elastic electron scattering from 1,4-dioxane in the energy range of 0-1000 eV. Absolute differential and integral elastic cross sections have been measured using a crossed electron-molecule beam spectrometer and the relative flow technique, at four energies in the 10-30 eV range and for scattered electrons in the angular range 20°-129°. The measured cross sections are compared with results from R-matrix computations, at the static exchange plus polarization level, calculated at energies between 0-20 eV, and with calculations employing the independent atom model with the screening corrected additivity rule (IAM-SCAR). Those latter computations were conducted at energies between 1 and 1000 eV. Agreement between the measured and R-matrix cross sections was typically found to be good at all common energies, whereas agreement with IAM-SCAR was satisfactory only at 30 eV. To the best of our knowledge, the present results are the first absolute data to be published in the literature for this scattering system.

The absolute differential and integral electron excitation cross sections for the atomic oxygen ³P → ³D° transition (989 Å) have been measured at incident energies from 20 to 200 eV. A variable angle and incident energy electron spectrometer coupled to a microwave discharge free supersonic jet expansion source was used for the experiments. The source produced an O/O2 density ratio typically greater than 2.5. The differential cross sections were determined by comparison to computer‐fitted recalculations of the differential cross section for the atomic oxygen ³P →³S° transition (1304 Å). The method used for the computer fit and the results of the fitting calculations are discussed for the ³P →³S° atomic oxygen transition (1304 Å). The O I ³P →³S° (989 Å) differential cross sections vary smoothly with the incident energy and are flatter with scattering angle at the lower incident energies, as expected. The absolute integral cross section is found to agree with the theoretical shape at higher energies. Previous estimates of this cross section from fluorescence data were larger than the present values by approximately a factor of 2. The present lower value appears to provide much better agreement with aurora and dayglow 989 Å emission intensities.

Previously reported measurements of the absolute experimental electron excitation cross section for the O (³P → ³S°) transition (1304 Å) at 100 eV have been extended to a wide range of incident energies from 16.5 to 200 eV. Both the O I and H I (n = 2) cross sections were measured simultaneously, and the H I (n = 2) cross sections were compared with previous results as a test of experimental reliability. The O I differential cross sections vary smoothly with incident energy and are flatter with scattering angle at the lower incident energies, as expected. The absolute integral cross section is found to agree with the theoretical shape and magnitude at higher energies. Comparison is made between fluorescence cross sections of Zipf and Erdman and present direct electron excitation cross sections. At low energies the direct excitation cross section is roughly 50% of the fluorescence cross section, suggesting that the cascade contribution to 1304‐Å fluorescence from excitation of the 2³P and other states is about the same magnitude as direct excitation. Generalized oscillator plots of the 100‐, 150‐, and 200‐eV data demonstrate the applicability of the Born approximation. Present results for the H I (n = 2) integral cross sections agree with previous results within 15%.

We report absolute differential cross sections (DCSs) for elastic electron scattering from GeF(4). The incident electron energy range was 3-200 eV, while the scattered electron angular range was typically 15°-150°. In addition, corresponding independent atom model (IAM) calculations, within the screened additivity rule (SCAR) formulation, were also performed. Those results, particularly for electron energies above about 10 eV, were found to be in good quantitative agreement with the present experimental data. Furthermore, we compare our GeF(4) elastic DCSs to similar data for scattering from CF(4) and SiF(4). All these three species possess T(d) symmetry, and at each specific energy considered above about 50 eV their DCSs are observed to be almost identical. These indistinguishable features suggest that high-energy elastic scattering from these targets is virtually dominated by the atomic-F species of the molecules. Finally, estimates for the measured GeF(4) elastic integral cross sections are derived and compared to our IAM-SCAR computations and with independent total cross section values.

Ratios of elastic differential cross sections of CO2 to those of N2 have been measured at electron impact energies of 500, 800 and 1000 eV in the angular range of 5 degrees to 120 degrees , using a crossed electron-beam-molecular-beam geometry and the relative flow technique. These ratios have been multiplied by previously known N2 elastic differential cross sections to obtain elastic differential cross sections for CO2. At 500 eV, the authors' experimental results agree quite well with the absolute values of Bromberg (1974). Comparison has also been made with the theoretical results obtained in the renormalised multicentre potential model approach of Botelho et al. and shows an agreement within 20% in the entire angular range. The elastic integral and momentum transfer cross sections have also been determined from the differential cross sections.

In the present work we report cross sections for electron collisions with the isomers propene (C3H6) and cyclopropane (c-C3H6). Electron-scattering differential cross sections (DCS) are reported for measurements carried out for energies 1.5-100 eV and the angular range of 20 degrees-120 degrees. Elastic integral cross sections (ECS), DCS, and momentum-transfer cross sections (MTCS) are reported for calculations carried out using the Schwinger multichannel method with pseudopotentials for the energy range of 2.0-40 eV and angular range of 0 degrees-180 degrees. The resemblance of the pi* shape resonance in the cross sections, observed at 1.5-2.0 eV for propene, to those in C2H4 and C2F4 clearly points to the effect of the double bond in the molecular structures for these molecules. Below 60 eV, we observed clear differences in peak positions and magnitudes between the DCS, ECS, and MTCS for C3H6 and c-C3H6, which we view as the isomer effect.

We report on measurements of differential cross sections (DCSs) for electron impact excitation of a series of Rydberg electronic-states in α-tetrahydrofurfuryl alcohol (THFA). The energy range of these experiments was 20-50 eV, while the scattered electron was detected in the 10°-90° angular range. There are currently no other experimental data or theoretical computations against which we can directly compare the present measured results. Nonetheless, we are able to compare our THFA DCSs with earlier cross section measurements for Rydberg-state electronic excitation for tetrahydrofuran, a similar cyclic ether, from Do et al. [J. Chem. Phys. 134, 144302 (2011)]. In addition, "rotationally averaged" elastic DCSs, calculated using our independent atom model with screened additivity rule correction approach are also reported. Those latter results give integral cross sections consistent with the optical theorem, and supercede those from the only previous study of Milosavljević et al. [Eur. Phys. J. D 40, 107 (2006)].

Using a Beer–Lambert attenuation approach, we report measured total cross sections (TCSs) for positron scattering from vinyl acetate (C4H6O2) in the incident positron energy range 0.15–50 eV. In addition, we also report an independent atom model with screening corrected additivity rule computation results for the TCSs, differential and integral elastic cross sections, the positronium formation cross section and inelastic integral cross sections. The energy range of these calculations is 1–1000 eV. While there is a reasonable qualitative correspondence between measurement and calculation for the TCSs, in terms of the energy dependence of those cross sections, the theory was found to be a factor of ∼2 larger in magnitude at the lower energies, even after the measured data were corrected for the forward angle scattering effect.

We report absolute differential cross sections for elastic electron scattering from SiF4. The incident electron energy range is 1.5–200 eV, while the scattered electron angular range was from 15° to 150°. The absolute scale of the differential cross sections was set using the relative flow technique with helium as the reference species. Corresponding integral elastic and momentum transfer data have been derived from our differential measurements. As a part of this study, independent atom model calculations were also performed. Those computations were found to be in generally quite good accord with the experimental results at both the differential and integral levels. In addition, excitation function measurements at a scattering angle of 60° and in the energy range 1–14 eV for the ν3 (d-stretch) and ν4 (d-deformation) fundamental vibrational modes are reported. Finally, differential cross sections for all four fundamental modes (ν1, ν2, ν3, ν4) at the single electron energy of 7 eV are given. These vibrational excitation measurements, when coupled with the elastic angular distributions in the 5–8 eV energy range, demonstrate the presence of a t2-symmetry shape resonance.

We report absolute elastic differential cross sections for electron interactions with the C4F6 isomers, hexafluoro-1,3-butadiene (1,3-C4F6), hexafluoro-2-butyne (2-C4F6), and hexafluorocyclobutene (c-C4F6). The incident electron energy range is 1.5-200 eV, and the scattered electron angular range for the differential measurements varies from 15° to 150°. In all cases the absolute scale of the differential cross section was set using the relative flow technique, with helium as the reference species. Atomic-like behaviour in these scattering systems is shown here for the first time, and is further investigated by comparing the elastic cross sections for the C4F6 isomers with other fluorinated molecules, such as SF6 and CnF6 (n = 2, 3, and 6). We note that for all the six-F containing molecules, the scattering process for electron energies above 30 eV is indistinguishable. Finally, we report results for calculations of elastic differential cross sections for electron scattering from each of these isomers, within an optical potential method and assuming a screened corrected independent atom representation. The level of agreement between these calculations and our measurements is found to be quite remarkable in all cases.

We present a study of coherent pion production in neutrino-nucleus interactions using the formalism based on partially conserved axial current theorem which connects the neutrino-nucleus cross section to the pion-nucleus elastic scattering cross section. Pion-nucleus elastic scattering cross section is calculated using Glauber model which takes three inputs, nuclear densities, pion-nucleon cross section and απN which is the ratio of real to imaginary part of πN forward scattering amplitude, for which the parametrizations are obtained from measured data. We calculate the differential and integrated cross sections for charge and neutral current coherent pion production in neutrino (anti-neutrino)-nucleus scattering for a range of nuclear targets from light to heavy materials such as lithium, carbon, hydrocarbon, oxygen, silicon, argon, iron and lead. The results of these cross section calculations are compared with the measured data and with the calculations from the Berger-Sehgal model and GENIE package. There is an excellent agreement between the calculated and measured cross sections with Glauber model. While GENIE and Berger-Sehgal model give a good description of the data in the lower energy range the present calculations describe the data in all energy ranges. Predictions are also made for upcoming experiments like INO and DUNE in the coherent region of neutrino cross section.

We report experimental measurements of the absolute total cross sections (TCSs) for electron scattering from 1-butanol at impact energies in the range 80-400 eV. Those measurements were conducted by considering the attenuation of a collimated electron beam, at a given energy, through a gas cell containing 1-butanol, at a given pressure, and through application of the Beer-Lambert law to derive the required TCS. We also report theoretical results using the Independent-Atom Model with Screening Corrected Additivity Rule and Interference approach. Those results include the TCS, the elastic integral cross section (ICS), the ionization total ICS, and the sum over all excitation process ICSs with agreement at the TCS level between our measured and calculated results being encouraging.

Absolute, elastic differential cross sections (DCS) for electron-propane collisions have been measured over a range of scattering angles from 10 degrees to 130 degrees and at incident kinetic energies from 2 to 100 eV. The angular distributions were extrapolated to 0 degrees and 180 degrees by a nonlinear phase shift fit and integrated numerically to obtain the integral elastic and momentum transfer cross sections. The observed DCS agree approximately with a continuum multiple scattering calculation that uses only local potentials and also with a multichannel Schwinger variational calculation. The vibrationally inelastic DCS reveal a shape resonance with a peak at 7.5 eV that is characteristic of trapping by valence orbitals with C-H antibonding character and is identical with peaks encountered in electron-methane and electron-ethane scattering.

This work presents a hybrid computing technique for modeling the differential elastic cross section of both proton--proton ``pp'' and proton--antiproton ``pp(bar)'' collisions from high to ultra-high energy regions (from 13.9 GeV to 14 TeV) as a function of the center-of-mass energy ``s'' squared and four momentum transfer squared ``t''. We proposed a genetic algorithm (GA) and support vector regression (SVR) hybrid techniques to calculate and predict the ``differential elastic cross section'' of both ``pp'' and ``pp(bar)''. Our proposed GA-SVR hybrid model shows a good match to the available experimental data, as well as predicting the latest and future ``TOTEM'' experiments for differential elastic cross sections that are not utilized in the training phase. An extrapolation of the differential cross section to $\vert t\vert \to $0 gives the total cross section through the optical theorem. The model calculations and predictions for ``pp'' and ``pp(bar)'' total cross-sections over a wide collision energy ranging from ISR to LHC are given. All the results for the total cross-sections matched well with the last and the more recent measurements ($\sqrt{ s}$= 7 TeV and 8 TeV), as well as the results of other models for the future ``TOTEM'' measurements at ($\sqrt{ s}$ = 10 TeV and 14 TeV).

Absolute differential and integral cross section measurements for electron impact excitation of the dissociative B1, A1 and A1 electronic states of water are described. These are very important channels for the production of OH (X2Π, ν) radicals, as well as for understanding the origin of the atmospheric Meinel bands. The energy range of our measurements is 20–50 eV, with the angular range of the differential cross section data being 10°–90°. Where possible comparison of the present cross sections with the corresponding results from theoretical calculations is made, the level of agreement between them being found to be quite marginal.