Experimental and theoretical study of the ion-ion mutual neutralization reactions Ar++SFn- (n=6, 5, and 4)

Abstract

The ion-ion mutual neutralization reactions Ar++SFn--> Ar+SFn (n=6, 5, and 4) have been studied in a flowing afterglow-Langmuir probe (FALP) apparatus at 300 K and 1 Torr of He buffer gas. Electron concentrations and product ion fractions were measured, and neutralization rate constants of 4.0x10(-8), 3.8x10(-8), and 4x10(-8) cm(3) s(-1) for SF6-, SF5-, and SF4-, respectively, were derived, with uncertainties of +/- 25% (+/- 35% for SF4-). During the neutralization process, excited neutrals are generated that are able to dissociate to neutral fragments. In the case of SF6, the formation of SF5 and SF4, and similarly in the case of SF5, the formation of SF4 and SF3 were observed and quantified. The mechanism of primary and secondary reaction was analyzed in detail, and rate constants for the dissociative electron attachments e(-)+SF5 -> F-+SF4 (k=3x10(-9) cm(3) s(-1),+/- 40%) and e(-)+SF3 -> F-+SF2 (k=2x10(-8) cm(3) s(-1),+400%,-75%) were also derived. The experimental ion-ion neutralization rate constants were found to be in good agreement with estimates from an optimum two-state double-passage Landau-Zener model. It was also found that energy partitioning in the neutralization is related to the extent of electronic excitation of Ar generated by the electron transfer processes. (C) 2008 American Institute of Physics.