On the accuracy of thermionic electron emission models. I. Electron detachment from SF6-

Abstract

Detailed statistical rate calculations combined with electron capture theory and kinetic modeling for the electron attachment to SF6 and detachment from SF6- [Troe et al., J. Chem. Phys. 127, 244303 (2007)] are used to test thermionic electron emission models. A new method to calculate the specific detachment rate constants k(det)(E) and the electron energy distributions f(E, epsilon) as functions of the total energy E of the anion and the energy epsilon of the emitted electrons is presented, which is computationally simple but neglects fine structures in the detailed k(det)(E). Reduced electron energy distributions f(E, epsilon/<epsilon > were found to be of the form (epsilon/<epsilon >)(n) exp(-epsilon/<epsilon >) with n approximate to 0.15, whose shape corresponds to thermal distributions only to a limited extent. In contrast, the average energies f(E, epsilon/<epsilon >) can be roughly estimated within thermionic emission and finite heat bath concepts. An effective temperature T-d (E) is determined from the relation E-EA= < ESF6(T-d)>+kT(d), where < E-SF6(T-d)> denotes the thermal internal energy of the detachment product SF6 at the temperature T-d and EA is the electron affinity of SF6. The average electron energy is then approximately given by <epsilon(E)>=kT(d)(E), but dynamical details of the process are not accounted for by this approach. Simplified representations of k(det)(E) in terms of T-d(E) from the literature are shown to lead to only semiquantitative agreement with the equally simple but more accurate calculations presented here. An effective "isokinetic" electron emission temperature T-e(E) does not appear to be useful for the electron detachment system considered because it neither provides advantages over a representation of k(det)(E) as a function of T-d(E), nor are recommended relations between T-e(E) and T-d(E) of sufficient accuracy. (C) 2009 American Institute of Physics. [DOI: 10.1063/1.3149782]