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 SF 6 and detachment from SF 6 − ͓Troe et al., J. Chem. Phys. 127, 244303 ͑2007͔͒ are used to test thermionic electron emission models. A new method to calculate the specific detachment rate constants k det ͑E͒ and the electron energy distributions f͑E , ͒ as functions of the total energy E of the anion and the energy of the emitted electrons is presented, which is computationally simple but neglects fine structures in the detailed k det ͑E͒. Reduced electron energy distributions f͑E , / ͗͒͘ were found to be of the form ͑ / ͗͒͘ n exp͑− / ͗͒͘ with n Ϸ 0.15, whose shape corresponds to thermal distributions only to a limited extent. In contrast, the average energies ͗͑E͒͘ can be roughly estimated within thermionic emission and finite heat bath concepts. An effective temperature T d ͑E͒ is determined from the relation E −EA=͗E SF 6 ͑T d ͒͘ + kT d , where ͗E SF 6 ͑T d ͒͘ denotes the thermal internal energy of the detachment product SF 6 at the temperature T d and EA is the electron affinity of SF 6 . The average electron energy is then approximately given by ͗͑E͒͘ = kT d ͑E͒, but dynamical details of the process are not accounted for by this approach. Simplified representations of k det ͑E͒ in terms of T d ͑E͒ 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 ͑E͒ does not appear to be useful for the electron detachment system considered because it neither provides advantages over a representation of k det ͑E͒ as a function of T d ͑E͒, nor are recommended relations between T e ͑E͒ and T d ͑E͒ of sufficient accuracy.

Threshold behavior and analytical fitting of partial wave capture probabilities for attractive R−n potentials

“…We, therefore, recommend the use of eqn (11) and (23) for l = 0 and n = 4 and 6, respectively. For l = 1-4 on the other hand, eqn (19) is recommended with g l (k) defined by eqn (18) and H l (k) taken from eqn (21) with the coefficients given in Table 1 and 2 for n = 4 and n = 6, respectively. The given approximate analytical expressions are of considerable practical use.…”

“…with a fitting monotonically-increasing function H c (k) that vanishes at k = k (1/2) c and guaranties the correct asymptotics g c (k)| kc1 -1. (The function g c (k) in eqn (18) is equivalent to the function G c (k) introduced in the Appendix of ref. 3 provided that the misprinted term 0.5 is removed from the r.h.s.…”

“…of the expression that defines G c (k)). With expression (18), the condition in eqn (5) is automatically satisfied, and the only remaining problem is a satisfactory matching of expressions in eqn (14) and (18) through a certain function P app c at an intermediate point k = k* c . We have found that the matching is achieved by piece-wise definition of P app c (k) whose explicit form,…”

Analysis by kinetic modeling of the temperature dependence of thermal electron attachment to CF3Br

“…A separation into P IVR el and P VEX el would only be essential when attachment and detachment properties are to be linked by detailed balancing. 18 Using the experimental data for DEA to CF 3 Br as first example, our article hopes to motivate further tests of the validity of Eq. (1.5).…”

Experimental and modeling study of thermal rate coefficients and cross sections for electron attachment to C60

“…Likewise, attachment cross sections and dissociative lifetimes of energy-selected anions are coupled by detailed balancing and need to be internally consistent. [6][7][8][9] Having demonstrated a combined experimental/ theoretical analysis of electron attachment/detachment in the SF 6 / SF 6 − system, it appears attractive to treat electron attachment to the much larger molecule C 60 in a similar fashion. A series of interesting differences are expected.…”