Determination of a set of electron impact cross sections in tetrafluoromethane consistent with experimental determination of swarm parameters

A set of electron-molecule cross sections in CF4 has been derived using the solution of the Boltzmann equation under the hydrodynamic regime. The set is evaluated by fitting the calculated swarm parameters to the experimental data. The obtained set is consistent with measured swarm parameters (drift velocity, transversal diffusion coefficient, ionization, and attachment coefficients), not only in pure gas but also in argon-CF4 mixtures as well with the data available. Information about cross sections (form and magnitude) coming from direct determination has also been respected, even though it is scarce. In mixtures, the electron swarm parameters drastically change even when a small amount of CF4 is added, showing a negative differential conductivity in the drift velocity. This phenomenon is studied in detail. Furthermore, the introduction of the superelastic collisions in the calculations is also justified. In the gas under study, due to the importance of the vibrational cross sections at low energy, the anisotropy of the electron energy distribution function is so high that it must be taken into account in the method used for solution of the Boltzmann equation. The validity of the set of cross sections (including elastic momentum transfer, vibrations, dissociation, ionization, and attachment) has been tested by comparing calculated and measured swarm data not used in the unfolding procedure. (C) 1996 American Institute of Physics.