Negative Ion Chemistry of Ozone in the Gas Phase

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 (O 3 ). This is the most comprehensive set of O 3 reactions with negative ions to date. The reactions proceed primarily through charge transfer and O atom transfer. The reaction rate constants for O -, O 2 -, 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 NO 2 -, CO 4 -, SF 6 -, and PO 2 - are somewhat less reactive, reacting at approximately 20−50% of the thermal capture rate. The hydrofluorocarbon ions CF 3 - and C 2 F 5 - react at 80% of the thermal capture rate, and F - is the major product ion formed. NO 3 -, CO 3 -, PO 3 -, CF 3 O -, F -, Cl -, and Br - are found to be unreactive with rate constants < 5 × 10 -12 cm 3 s -1, which is the present detection limit of our apparatus using this ozone source. The I - ion was observed to cluster with O 3 to form IO 3 - with a rate constant of approximately 1 × 10 -11 cm 3 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 cm 3 s -1, with O 2 .