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Rate coefficients for the reaction between HO2 and ClO radicals were obtained using the discharge-flow/mass-spectrometry technique at total pressures of 1.1−1.7 Torr of He, and between 298 and 215 K. The room-temperature rate constant, determined using seven different combinations of HO2 and ClO precursors, was found to be (7.1 ± 1.8) × 10-12 (±2σ) cm3 s-1. The temperature-dependent overall reaction rate coefficient is described by k (3) (298−215 K) = (7.1 ± 0.4) × 10-12 exp(−16 ± 17/T) cm3 s-1. The previous observation of a strong negative temperature dependence in the title reaction below 298 K was not observed, resulting in a significantly lower rate coefficient at stratospheric temperatures. HOCl was the only product of the reaction, and an upper limit of 1% for the branching ratio for the formation of O3 and HCl was obtained at the low pressures of these experiments.

The branching ratio for the OH + ClO reaction has been measured using the turbulent flow technique with high-pressure chemical ionization mass spectrometry for the detection of reactants and products. In our earlier study, OD was used instead of OH due to the large HCl background produced by the ClO source (J. Chem. Soc., Faraday Trans. 1997, 93, 2665. Improvements to our experimental technique have significantly reduced this HCl background, thus making it possible to observe directly the production of very small concentrations of HCl (∼10 9 molecule cm -3 ) from the minor channel of the OH + ClO reaction. At room temperature and ∼100 Torr pressure, the rate constant for this minor channel was determined to be (9.5 ( 1.6) × 10 -13 cm 3 molecule -1 s -1 with a two standard deviation error limit, which remained unchanged when the pressure was increased to 200 Torr. The temperature dependence of the rate constant for this minor channel was also investigated between 207 and 298 K, and the data were fit to the following Arrhenius expression: (3.2 ( 0.8) × 10 -13 exp[(325 ( 60)/T] cm 3 molecule -1 s -1 . The branching ratio for the HCl channel was determined to be 0.07 ( 0.03 at all pressures and temperatures investigated in this study. Statistical rate theory calculations were also performed on the OH + ClO reaction system and are in good agreement with the experimental results.

Fluorinated hydrocarbon radical-radical reactions in the gas phase have been studied at low pressure (0.5 Յ p/mbar Յ 2) and low temperature (253 Յ T/K Յ 333) using the discharge flow reactor molecular beam sampling mass spectrometry (MS) technique. Stable and labile species have been detected by MS applying low energy electron impact as well as multiphoton ionisation.For the combination reaction CH2F ϩ CH2F → products (1) the rate coefficient k 1 ϭ (7.0 Ϯ 0.8) · 10 12 (T/298) (Ϫ3.9Ϯ1.0) cm 3 /mol · s was determined. At low pressure the HF elimination pathway (1b)is the main channel (k(1b)/k1 ϭ (0.82Ϯ 0.015)). For the CHF2 radical self reaction CHF 2 ϩ CHF 2 → products (2) the rate coefficient was measured as k 2 ϭ (1.7 Ϯ 0.5) · 10 13 exp((Ϫ555 Ϯ89)/T) cm 3 / mol · s. The stabilisation of C 2 H 2 F 4 * is the main reaction pathway (2a) (k 2a /k 2 Ն 0.5), the HF elimination is of minor importance (k 2b/k2 ϭ 0.2). No pressure dependence of k1 and k2 was observed in the limited pressure range.

Elementary reactions / CF2(X ) / O3 / Mass spectrometryThe reaction of CF 2 (X ) with O 3 has been investigated in a discharge flow reactor at room temperature and low pressure (p ϭ 2.5 mbar). CF 2(X ) was produced by a microwave discharge of a mixture of CF2Br2/He. The reactants and products were detected by mass spectrometers (MS), which were connected to the flow systems via a continuous molecular beam sampling system.The following primary products were observed (CF 2 O 3 (m/z ϭ 98); CF 2 O (m/z ϭ 47, 66); O 2 (m/z ϭ 32)). The reaction mechanism:explains the observed primary products. The molecule with the empirical formula CF 2O3 has been observed directly by MS; the structure is discussed.