Philippe Dagaut

Kinetic measurements of the gas phase HO2+CH3O2 cross-disproportionation reaction at 298 K

Abstract Flash photolysis kinetic absorption spectroscopy was used to investigate the gas phase reaction between hydroperoxy (HO 2 ) and methylperoxy (CH 3 O 2 ) radicals at 298 K. Due to the large difference between the self-reactivities of the two radicals, first- or second-order kinetic conditions could not be maintained for either species. Thus, the rate constant for the cross reaction was determined from computer-modeled fits of the radical absorption decay curves, at wavelengths between 215 and 280 nm. This procedure yielded k = 2.9 × 10 −12 cm 3 molecule −1 s −1 independent of total pressure (using N 2 ) between 25 and 600 Torr, and of the partial pressure of water vapor (up to 11.6 Torr). There was also no effect of water vapor on the rate constant for the self-reaction of methylperoxy radicals.

Measurements of the gas phase UV absorption spectrum of C2H5O2•radicals and of the temperature dependence of the rate constant for their self-reaction

Abstract A flash photolysis technique has been used to measuer the gas phase UV absorption cross-sections for C 2 H 5 O 2 · radicals over the wavelength range 215 – 300 nm. Kinetic absorption spectroscopy was then employed to study the self-reaction of these radicals. The measured absorption cross-section at 250 nm, (3.89 ± 0.54) × 10 −18 cm 2 molecule −1 , was sued to derive an observed self-reaction rate constant at room temperature, defined as −d[C 2 H 5 O 2 ]·/d t = 2 k obs [C 2 H 5 O 2 ·] 2 , of k obs = (9.87 ± 0.74) × 10 −14 cm 3 molecule −1 s −1 , independent of pressure over the range 25 – 400 Torr. Experiments were performed over the temperature range 228 – 380 K and the kinetic data were fit by the Arrhenius expression ( k obs = (1.41 ± 0.19) × 10 −13 exp{su−(110 ± 40)/ T } cm 3 molecule −1 s −1 ) where the error limits represent 2σ from linear least-squares analysis. A modeling assessment of the effects of secondary reactions involving C 2 H 5 O 2 · radicals indicates that these measured values for k obs could be higher than the true rate constant for the self-reaction by as much as a factor of 1.7 depending on the branching ratio for the various product channels for the reaction C 2 H 5 O 2 · + C 2 H 5 O 2 · → Products. Thus k 1 ≈ (0.6) k obs .

A flash photolysis investigation of the UV absorption spectrum and self-reaction kinetics of CH2ClCH2O2 radicals in the gas phase

Abstract The ultraviolet absorption spectrum of the chloro-ethylperoxy radical, CH 2 ClCH 2 O 2 , and the kinetics of its self-reaction have been studied in the gas phase using a flash photolysis technique. The room temperature absorption cross section at 250 nm was determined to be σ CH 2 ClCH 2 O 2 (250 nm)=(3.64±0.39)×10 −18 cm 2 molecule −1 and was used to calculate an observed self-reaction rate constant, defined as -d[CH 2 ClCH 2 O 2 ]/d t =2 k obs [CH 2 ClCH 2 O 2 ] 2 , of k obs (298 K)=(3.57±0.57)×10 −12 cm 3 molecule −1 s −1 , independent of pressure over the range 25–400 Torr. Data over the temperature range 228–380 K and at a total pressure of 100 Torr were used to obtain the Arrhenius expression k obs =(1.1±0.7)×10 −13 exp[(1020±170)/ T ] cm 3 molecule −1 s −1 .

The gas phase UV absorption spectrum of CH3O2 radicals: A reinvestigation

Abstract Computational analyses of the effects of possible HO 2 interferences on our earlier reported results for the CH 3 O 2 UV spectrum and self-reaction kinetics are presented. New absorption cross-section data for CH 3 O 2 are derived from experiments in which secondary formation of HO 2 is virtually excluded. The results are discussed in terms of the literature data.

Gas phase studies of substituted methylperoxy radicals: the UV absorption spectrum and self-reaction kinetics of CH3OCH2O2 — the reaction of CF2ClO2 with Cl atoms

Abstract A flash photolysis technique has been used to measure the gas phase UV absorption cross-sections for the methoxy-methylperoxy radical, CH 3 OCH 2 O 2 , over the wavelength range 210 – 290 nm. Reaction (1) was then studied by kinetic absorption spectroscopy and the rate constant, defined as —d[CH 3 OCH 2 O 2 ]/d t = 2 k 1 [CH 3 OCH 2 O 2 ] 2 , was derived using σ CH 3 OCH 2 O 2 (240 nm) = (3.65 ± 0.35) × 10 −18 cm 2 molecule −1 . The self-reaction rate constants were observed to be in the fall-off region over the temperature range 228 – 380 K at pressures between 25 and 800 Torr (using N 2 ) and were fit using the formulation developed by Troe for association reactions. Attempts at similar studies for CF 2 ClO 2 radicals were hindered by the apparent secondary formation of ClO. An interpretation of the experimental observations in this radical formation system is offered which is consistent with the occurrence of a rapid reaction between CF 2 ClO 2 and Cl yielding ClO

Energy transfer from vibrationally excited SF6 to benzene, hexafluorobenzene, fluorobenzene and toluene

Abstract Relaxation of vibrationally excited SF6 by the two distinct processes, V → R,T and V → V, was studied using the various large-molecule deactivators benzene, hexafluorobenzene, fluorobenzene and toluene. Energy relaxation rates were measured using two methods, one sensing changes in the translational energy of the gas mixture, the other sensing changes in the internal energy of the deactivating molecule. Collision efficiencies and estimates of accuracy for the V → R,T and V → V energy transfer processes were determined through detailed modeling. Results are compared with previous studies in which the time-resolved vibrational energy in the donor molecule was measured, but an unambiguous distinction between V → R,T and V → V processes could not always be made.

Energy transfer from vibrationally excited pentafluorobenzene to helium, xenon and water vapor

Abstract Pulsed infrared laser irradiation was used to excite pentafluorobenzene (PFB) in mixtures with helium, argon and water vapor. Measurements of the rates of energy transfer were made as a function of the initial laser excitation energy, total pressure, and composition of the gas mixture using the Hg tracer technique. The results are discussed with respect to the transfer of energy between vibrational, rotational, and translational degrees of freedom.