Communication: Transfer ionization in a thermal reaction of a cation and anion: Ar+ with Br- and I-

We present experimental evidence that reactions of argon cations Ar(+) with the halogen anions Br(-) and I(-) do not occur exclusively by mutual neutralization, but also produce the cations Br(+) or I(+) ions by transfer ionization (TI). The experiments were carried out in flowing-afterglow plasmas at gas temperatures between and 300 and 500 K, and employed a variant of the Variable Electron and Neutral Density Attachment Mass Spectrometry method. The measured TI rate coefficients are 1.9 ± 0.6 × 10(-9) cm(3) s(-1) and 1.1 ± (0.3)(0.8) × 10(-9) cm(3) s(-1) for the Br(-) and I(-) reactions, respectively. We find that the TI rate coefficients decline with temperature as T(-0.5) to T(-1). No indication of TI was found in the reaction with Cl(-), where it is endoergic.