Electron-neutral collision cross sections for H₂O: II. Anisotropic scattering and assessment of the validity of the two-term approximation

Abstract

This work proposes a complete and consistent set of cross sections for electron collisions with water gas molecules to be published in the IST-Lisbon database on LXCat. The set is validated by the electron swarm analysis technique. The anisotropic angular distribution of electrons in rotational collisions is considered by means of the Born approximation in a two-term Boltzmann solver
(LoKI-B) and a Monte Carlo simulations code (LoKI-MC), both freely available as open-source codes. The inclusion of electron anisotropic scattering in rotational collisions significantly improves the agreement between calculations and measurements of the electron drift velocity, reduced mobility, characteristic energy, reduced Townsend ionisation coefficient, reduced effective Townsend
coefficient and reduced attachment coefficient. The MC simulations are deemed more accurate and shown to yield similar results as LoKI-B with the proposed set. The use of LoKI-MC also validates the set of cross sections against parameters that cannot be obtained by LoKI-B, such as the longitudinal diffusion coefficient or the bulk transport coefficients.

Effect of the magnetic field on the electron kinetics under AC/DC electric fields: benchmark calculations and electron cyclotron resonance

“…Depending on the collision type, the change of the velocity vector is calculated based on the generation of random numbers and the conservation laws for energy and momentum (see sections 2.5 and 2.6 of [11] for more details). The angular scattering in any collision type can be either isotropic or anisotropic, as detailed in [15].…”

“…The LisbOn KInetics Monte Carlo (LoKI-MC) was recently developed by our group, aiming to provide the community a general and flexible electron kinetics solver that does not depend on the two-term approximation, distributed as open-source [11]. Some of its interesting features are: (i) straightforward insertion of gas internal levels (electronic, vibrational and rotational) and, accordingly, of superelastic and stepwise processes; (ii) possibility of describing anisotropic scattering in any collision kind [15]; (iii) inclusion of the thermal motion of the gas molecules, allowing to describe correctly the electron swarm kinetics at low E/N; (iv) calculation of the electron power balance over the various collisional channels.…”

Benchmark calculations for anisotropic scattering in kinetic models for low temperature plasma

“…The solid angle integral in (9a) yields the same as what is defined as the 'partial cross-section' by Phelps and Pitchford [14] and employed by others [19,20]. Equation (9a) can be further decomposed using the relation σ(ε, χ) = σ(ε)I(ε, χ), to yield an expression for partial angular scattering and appropriate scattering-in expression per ℓ…”

“…Various approaches to anisotropic scattering have been proposed over the years (for examples, see [12]), and the proper treatment of anisotropic scattering continues to be an active area of research even today. For instance, the use of an anisotropic scattering model based on a dipole-Born approximation for rotational collisions was recently shown to significantly impact calculated electron transport coefficients in CO [19] and H 2 O [20].…”

“…We note that some benchmark calculations for simplified energy-independent models are available from the works of Reid [24], Haddad et al [25], and Leyh et al [26]. Recent works that report the effects of anisotropic scattering in real gases [13,19,20] are too complex to serve as effective benchmarks for the wider community because they require the simultaneous correct implementation of other physics, such as superelastic collisions.…”