The coupling of electron thermalisation and electron attachment; SF₆and CCl₄in rare-gas moderators

A detailed study of the attachment of electrons to an electronegative gas in the presence of a large excess of inert-gas moderator is carried out. The electron attachment process affects the electron thermalisation and vice versa; that is, thermalisation and reaction are coupled processes. In this paper the electron velocity distribution is determined with an eigenfunction expansion solution of the appropriate (two-term) Fokker-Planck or Boltzmann equation. The reciprocals of the eigenvalues of the Fokker-Planck operator are the characteristic relaxation modes for the approach of this reactive system to equilibrium. The lowest eigenfunction and eigenvalue describe the long-time behaviour whereas the higher eigenvalues and eigenfunctions describe the transient behaviour of the distribution function on a shorter timescale. The time dependence of the electron energy, density and the attachment rate coefficient are considered for SF 6 and CCl 4 separately, in the rare-gas moderators. The conditions for which the processes of thermalisation and electron attachment are coupled are considered in detail in terms of the changes in the eigenvalue spectrum of the Fokker-Planck operator with an increase in the concentration of the electronegative gas.