Ion Decay Studies in Photo-Ionized Bromine

The loss processes of ions observed in photo-ionized Br₂ have been studied by determining the time dependence of the ion number densities in the afterglow using quadrupole mass analysis techniques. The most abundant initially observed negative ion was Br^-. This species was found to convert to Br₃^- in the afterglow by the three-body reaction Br^-+2Br₂{→} Br₃^- +Br₂. The coefficient for the process was found to be kc=2.9×10^-29 cm⁶/sec at 296 ^∘{}K. The negative-ion---positive-ion ambipolar diffusion coefficients (D_a-p) for Br^- and Br₃^- were found to be 12.9 and 7.5 cm² Torr/sec, respectively. The Br₃^- species was found to decay mainly by ion ambipolar diffusion. The positive-ion species Br⁺, Br₂⁺, and Br₃⁺ were observed in the afterglow. Of these, the Br₂⁺ ion was initially most abundant. Although a rapid reaction involving Br⁺ takes place in the early afterglow, Br₂⁺ and Br₃⁺ decay mainly by diffusion. Dₐ₊p for the Br₂⁺ was determined to be 12.6 cm² Torr/sec for the case where the electron number density has decayed to zero and the predominant negative ion was Br^-. The ambipolar diffusion coefficient for Br₃⁺ was found to be essentially identical to that for the Br₃^- ion under the conditions where the Br₃⁺ ion and Br₃^- ion were the predominant ion species present.