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.