As part of a systematic study of approximations commonly made in solutions of the Boltzmann equation for electrons in molecular gases, we have investigated the effects of anisotropic scattering on electron transport coefficients in ${\mathrm{N}}_{2}$ and have extended our study of the multiterm expansion technique to higher E/n. A critical survey of published data yields a set of differential and integral cross sections for electron energies from 0.003 to ${10}^{4}$ eV. For electric-field--to--gas-density ratios E/n between 10 and 500 Td (1 Td${=10}^{\mathrm{\ensuremath{-}}21}$ V ${\mathrm{m}}^{2}$), the changes in the commonly measured transport and reaction coefficients resulting from the introduction of anisotropic elastic and inelastic scattering, while keeping the elastic momentum-transfer cross section constant, are less than 1%.These calculations were made with use of the multiterm spherical-harmonic expansion solution of the Boltzmann equation. For 500lE/nl1500 Td the changes in scattering anisotropy cause changes in transport and reaction coefficients which increase with E/n to about 10%. The errors in drift velocity, mean energy, and the reaction coefficients resulting from the use of the two-term spherical-harmonic expansion rather than a six-term expansion are less than 3% at 1500 Td.However, the errors in the diffusion coefficients become large (g25%) at our highest E/n. The calculated transport coefficients are in generally good agreement with experiment for E/n less than 300 Td, but the differences increase at higher E/n. The importance of proper interpretation of ionization and excitation experiments at high E/n is illustrated by calculations which model either an exponential growth of density in time or an exponential growth with position. The calculated ionization coefficients are low compared to most experiments for E/n less than 200 Td. At E/ng600 Td the agreement is good for the spatial growth experiments, but the calculated values are below experiment from the temporal growth experiments. The calculated excitation coefficients are generally higher than experiment at low and high E/n but in agreement with experiment at E/n near 150 Td.

Anisotropic scattering of electrons by N2 and its effect on electron transport.

Electron–molecule collision calculations using the R-matrix method

Electron scattering by molecules II. Experimental methods and data

Cross section data have been compiled for electron collisions with hydrogen molecules based on 71 references. Cross sections are collected and reviewed for total scattering, elastic scattering, momentum transfer, excitations of rotational, vibrational, and electronic states, dissociation, ionization, emission of radiation, and dissociative attachment. For each process, the recommended values of the cross section are presented for use. The literature has been surveyed through the end of 2006.

Cross Sections for Electron Collisions with Hydrogen Molecules

A tomographic inversion is used to analyze measurements of the auroral atomic oxygen emission line at 6300 A made by the atmosphere explorer visible airglow experiment. A comparison is made between emission altitude profiles and the results from an electron transport and chemical reaction model. Measurements of the energetic electron flux, neutral composition, ion composition, and electron density are incorporated in the model.

The auroral 6300 Å emission: Observations and modeling

Total scattering cross sections have been measured in the same apparatus for positrons and electrons colliding with ${\mathrm{H}}_{2}$, ${\mathrm{N}}_{2}$, and C${\mathrm{O}}_{2}$ using a beam transmission technique. The projectile impact energies range from 1 - 500 eV for ${e}^{+}\ensuremath{-}{\mathrm{H}}_{2},2\ensuremath{-}500$ eV for ${e}^{\ensuremath{-}}\ensuremath{-}{\mathrm{H}}_{2},0.5\ensuremath{-}750$ eV for ${e}^{+}\ensuremath{-}{\mathrm{N}}_{2},2.2\ensuremath{-}700$ eV for ${e}^{\ensuremath{-}}\ensuremath{-}{\mathrm{N}}_{2},0.5\ensuremath{-}60$ eV for ${e}^{+}\ensuremath{-}{\mathrm{CO}}_{2}$, and $2\ensuremath{-}50$ eV for ${e}^{\ensuremath{-}}\ensuremath{-}{\mathrm{CO}}_{2}$. The onset of positronium formation is clearly seen by an abrupt rise in the total cross sections for positrons colliding with each of the molecules at the respective positronium-formation thresholds. The positron measurements are compared with the electron measurements at intermediate energies for ${\mathrm{H}}_{2}$ and ${\mathrm{N}}_{2}$. This comparison reveals a merging of the cross sections for ${\mathrm{H}}_{2}$ at energies above 200 eV, while for ${\mathrm{N}}_{2}$ the electron results remain higher than the positron results at all energies. Estimates are made of potential experimental errors, as well as the experimental resolution for discrimination against projectiles scattered at small forward angles.

Total-cross-section measurements for positrons and electrons colliding with H 2 , N 2 , and C O 2

Total scattering cross sections have been measured in the same apparatus for positrons and electrons colliding with helium, neon, and argon atoms in the energy range from 15 to 800 eV using a beam-transmission technique. These measurements reveal a merging of the positron and electron cross-section curves for helium at energies above 200 eV while the available theories predict this merging to occur at considerably higher energies. For neon and argon the positron and electron total-cross-section curves are slowly approaching each other at the highest energies. The present experimental approach is analyzed with regard to the discrimination against small-angle forward elastic scattering, and estimates are made of other potential errors in the measured total cross sections. The present results are used to test the zero-energy sum rule, obtained from forward dispersion relations, and it is found that these data are consistent with prior measurements in that the sum rule is found to fail for electron scattering and to be valid for positron scattering.

Measurements of total scattering cross sections for intermediate-energy positrons and electrons colliding with helium, neon, and argon

Total scattering cross sections have been measured for 0.3--31 eV positrons colliding with helium atoms and for 0.25--24 eV positrons colliding with neon atoms using a beam-transmission technique. These measurements have resulted in the first direct observations of Ramsauer-Townsend effects for positrons colliding wih helium and neon, and also provide clear indications of the onset of positronium formation in each of these gases. As an over all check of the reliability of the experimental approach for measuring total scattering cross sections for positrons, electron-atom scattering cross sections have been measured in the identical apparatus, using the same target gases, and the same technique as was used for the positron measurements.

Measurements of total scattering cross sections for low-energy positrons and electrons colliding with helium and neon atoms

Total scattering cross sections have been measured in the same apparatus for positrons and electrons of well-defined energies (0.35--100 eV) colliding with krypton and xenon using a beam-transmission technique. The positron total cross sections are largest at the lowest energies studied, and decrease in a manner very similar to the elastic scattering cross-section calculations by Schrader and by McEachran et al. The total cross sections for positron-krypton, xenon collisions experience abrupt changes in their slope at the respective positronium-formation thresholds indicating that positronium formation may be an important contributor to the total cross section above the formation threshold. The present electron measurements are in good agreement with the recent measurements of Wagenaar. Estimates of potential errors in the present measurements due to incomplete discrimination against small-angle forward elastic scattering are made.

Measurements of total scattering cross sections for low-energy positrons and electrons colliding with krypton and xenon

We describe and analyze a transmission experiment developed for measuring total scattering cross sections for low‐energy poistrons (0.3–250 eV) colliding with gases in a region containing a curved, axial magnetic field. It is determined that spiralling does not appreciably increase the beam path length through the curved, axial magnetic field in the gas scattering region. An overall test of the experimental apparatus and technique is made by measuring total cross sections for electrons colliding with helium and argon atoms in the energy range from 1.5 to 30 eV, where it is found that the present measurements are within 15% of the most reliable prior measurements.

V. Pol

Papers

Total-cross-section measurements for positrons and electrons colliding with H 2 , N 2 , and C O 2

Measurements of total scattering cross sections for intermediate-energy positrons and electrons colliding with helium, neon, and argon

Measurements of total scattering cross sections for low-energy positrons and electrons colliding with helium and neon atoms

Measurements of total scattering cross sections for low-energy positrons and electrons colliding with krypton and xenon

Transmission experiment for measuring total positron–atom collision cross sections in a curved, axial magnetic field