Experimental Kinetic Study of the Reactions of Fluorine Atoms with H2O and OH

Reactions of fluorine atoms with H2O and OH play an important role in atmospheric chemistry, the interstellar medium, and combustion processes. In this work, a discharge-flow reactor coupled with an electron-impact ionization mass spectrometer was used to investigate the kinetics of these reactions at a total helium pressure of 2 Torr and over a wide temperature range. The rate constant for the F + H2O reaction (1) was determined both by absolute measurements conducted under pseudo-first-order conditions, monitoring the kinetics of F-atom consumption in the presence of excess water, and by the relative-rate method (using the F + Br2 reaction as a reference): k1 = 3.3 × 10−12 × (T/298)1.15 exp(385/T) cm3 molecule−1 s−1 in the temperature range (225–960) K with a total estimated uncertainty of 15% at all temperatures. A U-shaped temperature dependence was observed for the reaction rate constant, consistent with earlier theoretical predictions. The rate constant for the reaction between F atoms and OH radicals (2) was determined from the decays of the OH concentration, monitored under conditions of excess fluorine atoms: k2 = (1.09 ± 0.04) × 10−11 exp((447 ± 0.12)/T) cm3 molecule−1 s−1 at T = 225–610 K, where the uncertainties reflect the statistical 2σ precision.