Elastic Integral Cross Sections Research Articles

Modeling dynamical correlation forces in low-energy positron scattering from polyatomic gases: a comparison for CH 4

Two different models for treating the contributions of dynamical electron–positron correlation forces to the full interaction between low-energy positron beams scattered off polyatomic gases are considered and applied to obtain the elastic integral and differential cross sections (at collision energies below Ps formation) for the methane molecule. The computed quantities are then compared with available experiments, both for integral and differential data, and found to be in good accord with measured quantities. Both models also agree rather closely with each other and the physical reasons for this behavior are briefly discussed.

Electron collisions with ethylene

We have measured absolute elastic scattering and vibrational excitation crosssections for electron impact on ethylene. The experimental data have beenobtained on two different crossed-beam electron spectrometers and they cover theenergy range from 1 to 100 eV and scattering angles between 10° and130°.Both differential (in angle) and energy-dependent cross sections have beenmeasured. The differential cross sections have also been analysed using amolecular phase shift analysis technique in order to derive the integral elastic andelastic momentum transfer cross sections. Comparison is made with earlier data,where available, and also with a number of recent theoretical calculations.

Low-energy positron dynamics in small hydrocarbon gases

In the present paper we report a set of parameter-free calculations for the scatteringof low-energy positrons off gaseous hydrocarbon molecules like C2H2, C2H4, C2H6 andC6H6.The calculations of the corresponding elastic integral cross sections have been carriedout within the framework of the symmetry-adapted, single-centre-expansionapproach to describe all the bound molecular electrons and the scattered positron,at energies below the thresholds for positronium formation. The correspondingmultichannel coupled equations have been solved using a Volterra-type set ofintegral equations.The computed cross sections are found to be fairly close to the existingexperimental data for all the molecules examined in our work, both in shape andvalue.We have also computed the positron annihilation coefficients,Zeff (T),at the experimentally observed temperature of 300 K and the effect of thebreathing vibration in computing the rates of annihilation for differentmolecular geometries is examined for the case of gaseous C2H2.The present results, among the first non-empirical calculations for polyatomic gases,confirm our earlier findings (Gianturco et al 2001 Phys. Rev. A 64 32715; Gianturcoand Mukherjee 1997 Phys. Rev. A 56 3638; 2001 Phys. Rev. A 64 024703) that theZeffvalues in such gases are crucially dependent on the inclusion of dynamicalcoupling between the positron motion and the nuclear vibrations during theannihilation event.

Elastic and transport cross sections for argon in hydrogen plasmas

Accurate elastic differential scattering cross sections have been computed through a fully quantum mechanical approach for hydrogen ions (H+, D+, or T+) colliding with argon in the center of mass energy range of 0.1 to 200 eV. With the appropriate weight functions, these differential cross sections have been integrated to yield the momentum transfer, viscosity, and integral elastic cross sections. The momentum transfer and viscosity cross sections have been extended to higher collision energies (up to 10 keV) using a classical, three-body scattering method. Results have been compared with those of a simple model and previous recommended values. Knowledge of these cross sections, tabulated and made available through the world wide web, is of use to modeling and diagnostics of gaseous electronic, fusion, and other plasma environments.

Theoretical study on electron-free radical collisions: An application to SiH and SiF

We present a theoretical study of electron collisions on SiH and SiF free radicals in the low- and intermediate-energy range. More specifically, calculated elastic differential, integral and momentum transfer cross sections as well as grand total (elastic and inelastic) and absorption cross sections are reported in the (1-500)-eV energy range. A complex optical potential is used to represent the electron-radical interaction while the Schwinger variational iterative method combined with the distorted-wave approximation is used to solve the scattering equations. Comparisons were made between the calculated cross sections for SiH and SiF as well as with the existing experimental and theoretical data for electron scattering by silane molecule. Some interesting aspects of this comparison are discussed here.

Total and elastic electron scattering cross sections from Xe at intermediate andhigh energies

Experimental total electron scattering cross sections from Xe in the energy range300–5000 eV have been obtained with experimental errors of about 3%. Themethod was based on the measurement of the attenuation of a linear electronbeam through a Xe gas cell in combination with an electron spectroscopytechnique to analyse the energy of the transmitted electrons. Differentialand integral elastic cross sections have been calculated using a scatteringpotential method which includes relativistic effects. The consistency of ourtheoretical and experimental results is also discussed in the paper. Finally,analytical formulae depending on two parameters, namely the number oftarget electrons and the atomic polarizability, are given to reproduce theexperimental data for Ne, Ar, Kr and Xe in the energy range 500–10 000 eV.

Electron-scattering cross sections of fluoromethanes in the energy range from 0.1 to 10 keV

Total electron-scattering cross section for ${\mathrm{CF}}_{4}$ has been measured in the energy range from 300 to 5000 eV. Semiempirical values for the total cross sections of ${\mathrm{CF}}_{4},$ ${\mathrm{CF}}_{3}\mathrm{H},$ ${\mathrm{CF}}_{2}{\mathrm{H}}_{2},$ and ${\mathrm{CFH}}_{3}$ between 0.5 and 10 keV are also given, obtained by means of a formula that depends only on the total number of electrons and the polarizability of the molecule, and which was previously deduced by the authors. Experimental and semiempirical values have been compared with previously published experimental data. Differential and integral elastic cross sections are given at several incident electron energies in the range from 100 to 10 keV, calculated by means of a scattering potential method, assuming that the independent-atom model is valid, and the results compared with other published data. Integral inelastic cross-section values are also given as the difference between the total cross section and the integral elastic cross-section data. Finally, in order to facilitate the use of the present results in computation work, our integral elastic and inelastic cross sections have been fitted to expressions based on Born-Bethe formulas, which allows one to obtain these cross sections at any energy within the energy range considered.

Theoretical study on electron–free-radical scattering: An application to CF

In this work, a theoretical study on electron-CF collisions in the low- and intermediate-energy range is reported. More specifically, calculated elastic differential, integral, and momentum-transfer cross sections as well as grand total (elastic and inelastic) and absorption cross sections are presented in the (1--500)-eV energy range. A complex optical potential is used to represent the electron-molecule interaction dynamics, while the Schwinger variational iterative method combined with the distorted-wave approximation is used to solve the scattering equations. Comparison of the present results with existing experimental and theoretical data for electron collisions with NO (an isoelectronic molecule of CF) is made.

He(11S)–He(23S)collision and radiative transition at low temperatures

There has been considerable recent interest in low-temperature interrogation of quantum helium nanodroplets. We consider the interaction of helium ground and metastable states, employing the best known molecular dimer potentials. Radiative transitions between different metastable rovibrational levels are compared with the discharge emission transition lines and good agreement is found. Collisional interactions between helium ground and metastable atoms are studied and elastic scattering, metastability excitation transfer and diffusion cross sections and rate coefficients are calculated at ultralow energies. The integral elastic cross section at ultralow energies is about $5000 {\mathrm{\AA{}}}^{2}.$ Effect of shape resonances that form in the potential barriers, on the cross sections is investigated. We find that some shape resonances live for longer than 100 s. Long-range potential wells could support ``shelf'' levels. We discuss their collisional and spectroscopic properties.

Elastic and absorption cross sections for electron–nitrous oxide collisions

In this work, we present a joint theoretical-experimental study on electron-${\mathrm{N}}_{2}\mathrm{O}$ collisions in the intermediate energy range. More specifically, calculated and measured elastic differential, integral, and momentum-transfer cross sections, as well as calculated total and absorption cross sections are reported. The measurements were performed using a crossed electron-beam--molecular-beam geometry. The angular distribution of the scattered electrons was converted to absolute cross sections using the relative-flow technique. Theoretically, a complex optical potential is used to represent the electron-molecule interaction dynamics in the present calculation. The Schwinger variational iterative method combined with the distorted-wave approximation is used to solve the scattering equations. The comparison of the present calculated results with the measured results as well as with the existing experimental and theoretical data shows good agreement.

Theoretical studies on electron-carbon monoxide collisions in the low and intermediate energy range

We report a theoretical study on electron–CO collisions in the low and intermediate energy range. More specifically, calculated elastic differential, integral and momentum transfer cross sections, as well as grand total (elastic+inelastic) cross sections are reported in the 2–500eV energy range. A complex optical potential is used to represent the electron–molecule interaction dynamics in the present calculation, while the Schwinger variational iterative method combined with the distorted-wave approximation is used to solve the scattering equations. The comparison of the present results with existing experimental and other theoretical data shows good agreement.

Total and elasticelectron scattering cross sections from ozone at intermediateand high energies

Total cross sections for electron scattering from O3molecules in the energy range 350-5000 eV have been measuredfor the first time. The experimental method used was based onthe measurement of the attenuation of a collimated electronbeam through an O3-O2 mixture in combination with useof an electron energy loss technique to determine the purity ofthe ozone sample. Differential and integral elastic crosssections have also been calculated using a scattering potentialin the framework of the independent-atom model. The presenttheoretical and experimental results are compared with earliercalculations available in the literature.

Elastic scattering of low-energy electrons by NF3

We present results of e--NF3 collision calculations forenergies up to 60 eV. To compute the elastic integral, differential,and momentum-transfer cross sections we have employed the Schwingermultichannel method implemented with norm-conservingpseudopotentials (Bettega M H F, Ferreira L G and Lima M A 1993 Phys. Rev. A 47 1111) within the fixed-nuclei static-exchangeapproximation. We have compared our results with the complex Kohnresults of Rescigno and with the experimental data of Boesten et al, and found them in general good agreement. We have found twoindependent shape resonances in the NF3 elastic integral crosssection, and through its symmetry decomposition we assigned them tothe A1 and E representations of the C3v group. Theexistence of a shape resonance has also been reported by Rescignoand also by Boesten et al, who assigned it to the Erepresentation of the C3v group. We have also discussedthe possible role of these shape resonances in the dissociativeattachment of NF3 by electron impact.

Computing positron annihilation in polyatomic gases: An exploratory study

The rates of positron annihilation in molecular gases are known to depend on the nanoscopic structural features of the ambient molecules. The aim of the present study is to explore the possible relationships that exist between some of the most salient molecular structural features and the ensuing positron annihilation rates at room temperature. Quantum dynamical calculations are applied to a broad variety of polyatomic targets using a parameter-free model interaction with the impinging positrons. The ${Z}_{\mathrm{eff}}$ values, as well as the integral elastic cross sections, are computed for such systems over the relevant range of collision energies. The dynamical treatment is shown to yield realistic values for the cross sections and to produce ${Z}_{\mathrm{eff}}$ values whose agreement with the few existing measured data varies greatly from one system to another.

Electron collisions with octafluorocyclobutane, c-C4F8

We present calculated cross sections for elastic and inelastic collisions of low-energy electrons with octafluorocyclobutane, c-C4F8. 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.

Elastic scattering of electrons by molecular oxygen

The absorption potential of Staszewska et al. (Phys. Rev. A 28 (1983) 2740) is known to underestimate the experimental values of the elastic differential, integral and momentum transfer cross sections. Hence, it has been modified and applied to study the elastic scattering of electrons on molecular oxygen at intermediate energies. A comparison of the elastic cross section results, obtained by using modified absorption potential, with the experimental data shows a significant improvement over the unmodified absorption potential results. The differential, integral and momentum transfer cross sections obtained using the modified absorption potential are in very good agreement with the experimental data.

Electron Interactions With c-C4F8

The limited electron collision cross-section and transport-coefficient data for the plasma processing gas perfluorocyclobutane (c-C4F8) are synthesized, assessed, and discussed. These include cross sections for total electron scattering, differential elastic electron scattering, partial and total ionization, dissociation into neutral fragments, and electron attachment, as well as data on electron transport, ionization, and attachment coefficients. The available data on both the electron collision cross sections and the electron transport coefficients require confirmation. Also, measurements are needed of the momentum transfer and elastic integral cross sections, and of the cross sections for other significant low-energy electron collision processes such as vibrational and electronic excitation. In addition, electron transport data over a wider range of values of the density-reduced electric field are needed. The present assessment of data on electron affinity, attachment, and scattering suggests the existence of negative ion states near −0.6, 4.9, 6.9, 9.0, and 10.5 eV.

Electron scattering from gaseous SF6: Comparing calculations with experiments

The dynamical observables associated with low-energy electron scattering from SF6 molecules in the gas-phase, e.g., elastic differential cross sections, integral elastic cross sections and momentum transfer cross sections, are computed using quantum methods and describing the full interaction between the molecule and the impinging electron without empirical parameters. The above quantities are obtained over an energy interval ranging from a few meV up to 100 eV and the results are compared with the available experiments. Various aspects of the theoretical method employed are analyzed in relation to their agreement with the experimental data discussed in this work.

Cross-sections for electron scattering by O 2 at intermediate and high energies, 0.1–10 keV

Cross-section data, both for elastic and inelastic electron collisions with molecular oxygen, in the energy range 0.1–10 keV, have been obtained in this study by combining theoretical and experimental methods. Accurate total cross-sections (within 3%) have been measured in a transmission beam system for electron energies ranging from 400 to 5500 eV. Differential and integral elastic cross-sections have been calculated by means of a scattering potential method in the framework of the independent atom model at electron energies ranging from 0.1 to 10 keV. The consistency of our theoretical and experimental results has been confirmed by comparison with previous elastic and ionisation experimental cross-sections.

The close-coupling-single center-expansion (CC-SCE) model for electron scattering from polyatomic targets

The theoretical and computational study of elastic integral and differential cross sections for fairly large polyatomic targets is described in terms of quantum close-coupled scattering equations cast within a symmetry-adapted, single center expansion form of target wavefunctions, continuum electrons, and interaction potential. The latter is introduced in a local, real model form, and examples of its level of agreement with experiments are given for a class of many-electron, polyatomic molecules like the halogenated silane derivatives. © 2001 John Wiley & Sons, Inc. Int J Quantum Chem, 2001