A new technique for studying ion-ion recombination in a flowing afterglow Langmuir probe apparatus: Ar+ recombining with Cl2−, CCl2O−, Br2−, SF5− and SF6−

We present a new technique for measuring ion-ion recombination rate constants in a flowing afterglow Langmuir probe (FALP) apparatus. The technique involves measuring the fractional negative ion product distribution following electron attachment versus the initial electron density when two or more products are formed. The concentration of reactant gas is kept low enough that the plasma retains its electron-Ar+, ambipolar diffusion character along the entire length of the flow tube. If only polyatomic anions are formed, accurate relative rates are obtained. When one of the species is atomic, absolute rates are also possible by doing a detailed model of the plasma kinetics. Here we present rate constants for Ar+ recombining with Cl2− ((5.3 ± 1.6) × 10−8 cm3 s−1 at 302 K), Br2− ((3.9 ± 1.2) × 10−8 cm3 s−1 at 302 K), the phosgene negative ion CCl2O− ((8.9 ± 2.7) × 10−8 cm3 s−1 at 302 K), and relative rate constants for Ar+ + SF6− and SF5− (ratio 1.2 at 550 K, with an uncertainty of +0.3 and −0.1). The diatomic negative ions are found to recombine slower than the polyatomic ones, in agreement with earlier indications.