Measurement of thermal electron dissociative attachment rate constants for halogen gases using a flowing afterglow technique

Abstract

A flowing afterglow apparatus was constructed and used to measure thermal electron attachment rate constants in the halogen gases fluorine, chlorine, and bromine. The operation of the afterglow system and the mathematical models applied in treating the data were tested by measuring the thermal attachment rate constant for electrons in sulfur hexafluoride. The average value obtained for this rate constant is 4.2±1.1×10⁻⁸ cm³ molecule⁻¹ sec⁻¹ when a microwave discharge was used as the electron source and 3.6±1.8×10⁻⁸ cm³ molecule⁻¹ sec⁻¹ when a filament was used as the electron source. The average electron temperature was estimated to be approximately 600 °K for the microwave discharge source and approximately 350 °K for the filament source. A charge transfer reaction between sulfur hexafluoride and the ion O₂⁻ was also investigated in the present study to further assess the operation of the flowing afterglow apparatus in the microwave discharge source configuration. The average rate constant obtained for this reaction is 3.7±0.4×10⁻¹¹ cm³ molecule⁻¹ sec⁻¹ for an estimated ion temperature of 300–325 °K. The average rate constants obtained for the dissociative attachment of electrons in fluorine are 4.6±1.2×10⁻⁹ cm³ molecule⁻¹ sec⁻¹ for an electron temperature of approximately 600°K and 3.1±1.2×10⁻⁹ cm³ molecule⁻¹ sec⁻¹ for an electron temperature of approximately 350 °K. The average rate constants obtained for dissociative electron attachment in chlorine and bromine are 3.7±1.7×10⁻⁹ and 1.0±0.9×10⁻¹¹ cm³ molecule⁻¹ sec⁻¹, respectively, for an electron temperature of approximately 350 °K. The rate constant for the three‐body attachment reaction Br⁻+Br₂+Ar to form Br₃⁻ was also measured and found to be 1.9±0.5×10⁻²⁸ cm⁶ molecule⁻² sec⁻¹.