Negative Ion Chemistry of Ozone in the Gas Phase

Abstract

A novel ozone source was developed to study the negative ion chemistry of ozone in the gas phase. Rate constants and product ion branching fractions are reported for 17 negative ion−molecule reactions involving ozone (O3). This is the most comprehensive set of O3 reactions with negative ions to date. The reactions proceed primarily through charge transfer and O atom transfer. The reaction rate constants for O-, O2-, and OH- are large and approximately equal to the thermal energy capture rate constant given by the Su-Chesnavich equation based on average dipole orientation theory. The negative ions NO2-, CO4-, SF6-, and PO2- are somewhat less reactive, reacting at approximately 20−50% of the thermal capture rate. The hydrofluorocarbon ions CF3- and C2F5- react at 80% of the thermal capture rate, and F- is the major product ion formed. NO3-, CO3-, PO3-, CF3O-, F-, Cl-, and Br- are found to be unreactive with rate constants < 5 × 10-12 cm3 s-1, which is the present detection limit of our apparatus using this ozone source. The I- ion was observed to cluster with O3 to form IO3- with a rate constant of approximately 1 × 10-11 cm3 s-1, which is a factor of 2 above our detection limit, and no other product channels were observed. All of the anions listed above showed no reactivity, k < 5 × 10-13 cm3 s-1, with O2.