Comparison of calculated and experimental thermal attachment rate constants for SF6 in the temperature range 200-600 K.

Abstract

Electron-attachment cross sections are calculated for the process ${e}^{\mathrm{\ensuremath{-}}}$+${\mathrm{SF}}_{6}$\ensuremath{\rightarrow}${\mathrm{SF}}_{6}$${\mathrm{}}^{\mathrm{\ensuremath{-}}}$ in the energy range 1--200 meV. An electron scattering approximation is used in which diatomiclike potential energy curves near the equilibrium ${\mathrm{SF}}_{6}$ ground state are constructed from recent spectroscopic data. Excellent agreement is found over the entire energy range with experimental attachment cross sections at a temperature of 300 K for s-wave (l=0) scattering. The same calculation, with appropriate adjustment of the thermal populations, is used to calculate attachment rate constants k(\ensuremath{\epsilon}\ifmmode\bar\else\textasciimacron\fi{}) in the range 50--600 K for both s- and p-wave scattering. Comparisons are made with four independent sets of measured rate constants in the range 200--600 K, after adding an experimental estimate of the attachment rate for the process ${e}^{\mathrm{\ensuremath{-}}}$+${\mathrm{SF}}_{6}$\ensuremath{\rightarrow}${\mathrm{SF}}_{5}$${\mathrm{}}^{\mathrm{\ensuremath{-}}}$+F. Good agreement is found with three sets of data, and poorer agreement with the fourth. The calculation shows that the true s-wave behavior of the rate constant, in which k(\ensuremath{\epsilon}\ifmmode\bar\else\textasciimacron\fi{}) is independent of \ensuremath{\epsilon}\ifmmode\bar\else\textasciimacron\fi{}, will be realized only at temperatures T less than 115 K, and that the observed constancy in the range 300--600 K is accidental. Theory predicts approximately a 9% increase in k(\ensuremath{\epsilon}\ifmmode\bar\else\textasciimacron\fi{}) as T is lowered from 300 to 200 K.