Swarm Measurement of Cross Sections for Dissociative Electron Capture in Heavy Water, Chlorobenzene, and Bromobenzene

Abstract

One established method for the measurement of the cross section for electron capture involves the introduction of small quantities of the gas under study (sample gas) into a carrier gas, which determines the distribution of electron energies for specified values of E/P. A summary of the available information on the distribution of electron energies in ethylene, nitrogen, and argon shows that for E/P values ranging from 0 to 2.0 V cm^—1 Torr^—1 the three carrier gases can be used to cover the important range of electron energies between thermal energy and 10 eV. This method has been used to study dissociative electron capture in D₂O, C₆H₅Cl, and C₆H₅Br. Measurements show that the values for the energy integrated dissociative capture cross sections are approximately 4.6×10^—18 cm² eV for D₂O, 1.4×10^—17 cm² eV for C₆H₅Cl, and 5.7×10^—17 cm² eV for C₆H₅Br occurring at electron energies near 6.5, 0.76, and 0.70 eV, respectively. That the low‐energy processes in C₆H₅Cl and C₆H₅Br correspond to dissociative capture has been confirmed by the appearance of Cl^— and Br^— in a negative‐ion time‐of‐flight mass spectrometer.