Abstract
Electron-scattering cross sections in methane are analysed in the very-low energy region. The correspondence between integral elastic, elastic differential and momentum transfer cross sections is checked via a novel approach to modified effective range theory, in order to determine the depth and position of the Ramsauer-Townsend minimum. Phase shifts of the two lowest partial waves are obtained explicitly and parameterized by four coefficients with the physical meaning of the scattering lengths and the effective ranges. Using recent experiments on vibrational cross sections performed over an extended (0–180°) angular range and comparing several theories, an agreement within 10% has been obtained between experimental total and present summed (elastic + vibrational) cross sections in the whole 0.1–2.0 eV energy range. An additional check for consistency is done using two-term Boltzmann analysis to derive electron diffusion coefficients. Calculated drift velocities and transversal diffusion coefficients at 0–10 Td reduced electric field agree within 5% with experiments.
Highlights
The discovery of the Ramsauer-Townsend (R-T) minimum, observed for electron scattering in Ar at about 0.3 eV, independently in swarm and beam electron scattering experiments [1,2], triggered the development of quantum wave theories
In CH4, the depth of the R-T minimum is masked in the experimental total cross section by a maximum of the vibrational excitation, of the two overlapping (v2 and v4) deformation modes
In this paper we show that for methane only the two lowest partial waves are sufficient for the explicit analysis of electron-methane collisional data below 2 eV of impact energy
Summary
The discovery of the Ramsauer-Townsend (R-T) minimum, observed for electron scattering in Ar at about 0.3 eV, independently in swarm and beam electron scattering experiments [1,2], triggered the development of quantum wave theories. The R-T minimum occurs in heavier atomic gases, where the scattering potential is strong enough to induce a π (modulo π) phase-shift in the s-partial wave at low impact energies, where contributions from other partial waves are still small. It was shown previously in pioneering work for argon [3] that including an empirical polarization potential is necessary to reproduce theoretically the R-T minimum. In the present work we re-analyse available experimental data on differential, integral and momentum transfer elastic cross sections for electron scattering by methane in the region of the R-T minimum. In this paper we show that for methane only the two lowest partial waves are sufficient for the explicit analysis of electron-methane collisional data below 2 eV of impact energy
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