Improved techniques of measuring accurate electron - molecule cross sections near threshold and over a large angular range

Abstract

Abstract. Methods to measure absolute differential elastic and vibrational excitation cross sections using an electron spectrometer with a magnetic angle changer are discussed. Emphasized are the need to by-pass drifts and to properly correct for the instrumental response function when angle of detection or electron energy are varied. The results are illustrated with cross sections in nitrogen, methane and neopentane. The good agreement of the results with the experimental data from other leading groups (taken mostly without the angle changer) and with some theories indicate that the measurements are reliable. The angle-integrated data agrees well with the results of electrontransport experiments. Good agreement with theory is obtained for nitrogen, even in the difficult low-energy region. Electronic excitation in rare gases is also briefly mentioned.

Cross Sections for Electron Collisions with Methane

“…MTCS derived in this way agree in the minimum region both with electron swarm parameters, as well as with recent beam measurements. 35 MTCS due to Sohn et al 42 in the region of R-T minimum can be affected, as stated by the authors, by higher errors; the cross sections at 0.5 eV were determined in different runs. 52 The comparison of the p-wave phase shifts between 0.2 and 0.5 eV (see 42 In the high energy range, the spread of experimental MTCS due to Cho et al, 38 Sakae et al, 40 and Iga et al, 39 which is beam-derived and usually uses theory to extrapolate differential cross sections, is about ±10%, and the exceptions are the MTCS of Vušković and Trajmar.…”

“…Therefore, experiments at high angles are important. Out of several data sources listed in Table 4, the measurements by Shyn and Cravens, 41 who used an electrostatic analyzer extending up to 168 • , and Allan 35 and Cho et al, 38 who used a magnetic-field angle-changer extending up to 180 • , sample high angles. Two sets of data by Tanaka and collaborators 36,43 differ in the spectrometers used, the normalization procedures, and extrapolation formula for differential cross sections.…”

Elastic Electron Scattering from Methane Molecule in the Energy Range from 50–300 eV

“…Therefore, understanding transport features of methane in plasma and interpretation of electron methane collisions in different physical processes plays a significant role in laboratory research in different fields of physics as well as in the investigation of properties atmospheres planets in Solar System. Finally, from a theoretical perspective and, because of its simplicity, methane can serve as a test molecule for advancing novel electron-molecule collision theories (e.g., Blanco et al [ 14 ], Allan [ 15 ]).…”

“…For low incident electron energies (below 50 eV), elastic electron scattering from methane molecule has been studied experimentally and theoretically equally extensively. To cite few most recent papers, Allan [ 15 ] measured absolute differential elastic (impact electron energies 0.4–20 eV) and vibrational excitation cross-sections using an electron spectrometer with a magnetic angle changer, which allows measurements of differential cross-sections (DCSs) for backward angles. Bettega et al [ 18 ] calculated DCSs and integral cross-sections (ICSs) for energies between 3 and 10 eV, using Schwinger multichannel method with pseudopotentials.…”

“…Mahato et al [ 33 ] obtained analytical expressions for the static potentials of electron scattering from methane using Gaussian wave functions and studied elastic scattering from 10 to 500 eV incident electron energies utilizing those static potentials along with exchange and polarization potentials. Song et al [ 17 ] presented recommended elastic DCSs and ICSs obtained by averaging other authors’ data [ 15 , 19 , 20 , 21 , 25 , 29 , 30 , 31 ]. List of experimental and theoretical work on DCS for elastic electron scattering from methane molecule, in energy range of our interest is shown in Table 1 .…”